Dual Wield vs 2H
Intro
Dual wielding can be more versatile than 2H it allows you to switch to a shield setup to back-up a dead tank or save yourself from a sudden big amount of damage.
However 2h weapons do carry a stronger auto attack a single auto attack can hit up to (in terms of Ability Damage values) 0%-154%, an average of 77% damage per tick since there can be no delays between auto attacks and abilities. If backed behind a damage-boosting ultimate, a single auto attack can do an immense amount of damage.
As an example, backed by Berserk, a single auto attack can hit 0%-308%, for an average of 104% damage per tick! It’s very powerful and certainly gives 2H an edge in DPS barring the different abilities that affect damage for Melee and Ranged.
The other advantage of 2H is for bosses like Dagganoth Kings or Kalphite Queen is its easier to switch weapon/combat style if they are all 2H you only need to equip one weapon instead of a main-hand and off-hand.
Melee
All Melee weapons have the same range – except for halberds (noxious scythe and Dragon Rider Lance included), which have an extra square of reach. This makes halberds the best choice for Meleeing certain bosses like Vorago, RotS, and Araxxor.
Melee obviously has the biggest changes between the two substyles. The main thing that dual wield Melee specializes in is single-target, high-dealing DPS, and 2h Melee is more AoE damage with good single-target DPS.
Decimate and Cleave do the same damage (188%) and have the same cooldown. Decimate does 244% against foes using a shield (PvP-only) while Cleave is an AoE in an arc around the target.
Destroy and Hurricane are big changes to each other though. Destroy is 37.6%-188% damage four times, totalling to 150.4%-752% (an average of 451.2% ability damage over 4.8 seconds, so 56.4% damage per tick). Hurricane is two hits of 66%-219% and 84%-161%, totalling to 150%-380% (an average of 265% ability damage over 1.8 seconds, so 88.3% damage per tick).
However, Destroy has more DPM compared to Hurricane considering it lasts longer, and other abilities used after Hurricane can’t compare with Destroy as well. Still, you can’t go wrong with both choices, but Hurricane is great for burst damage and AoE while Destroy is good for when you want to apply high amounts of damage on a single target.
Flurry versus Quake well, Flurry isn't very good. It's probably best no to use it, it’s 18.8%-94% on all enemies around you four times, totalling to 75.4%-376% ability damage (average of 225.7% ability damage over 4.8 seconds, so 28.2% ability damage per tick). Quake is completely superior, on the other hand, with much better damage per tick (43.8% ability damage per tick on average) and carries a Defence/armor drain that works even on stat drain-immune bosses. There is a Mutated Flurry its not to bad but just makes Flurry a little bit better then what it is already.
There aren’t much to talk about Ultimate's though. They’re all very bad to use in PvM, as thresholds are better with a lower cost and Ultimate's don’t do that much damage anyway. So the only 2 Ultimate's that have use are Berserk for a Berserk rotation which makes you susceptible to lots of damage for more DPS and Meteor. Meteor is a big hitting AOE its good or clearing out Multiple Targets that are near. It's really good in the last Phase of Twin Furies or just clearing lots of monsters like Abyssal Demons in slayer.
Overall, you really can’t go wrong with whatever style you go in terms of DPS. If you need AoE or an extra square of range, then 2h (more specifically, halberds) is best. Some bosses in RuneScape, if using Melee, might require you using a scythe to be even viable/usable like Vorago and RotS.
However, in the case of Melee, dual wield and 2h aren’t really meant to be compared against each other, but rather for complementing each other. In recent times, it is better to take both dual wield AND 2H Melee to make use of each other's different basic and threshold abilities, like Cleave/Decimate and Quake/Destroy.
Ranged
Unlike Melee and Magic, there is a lot of variances to distance reach for the amount of weapons available. Crossbows (dual wield) have a reach of 7 squares, while thrown weapons (dual wield) have a reach of 5 squares (with the exception of sagaie with 9 squares). In contrast, short bows (2H) have a range of 7 squares, 2H crossbows 9 squares, and the noxious longbow 9 squares. Ultimately 2H weapons have a longer range than dual wield, but whenever it means anything to you or not is up to you.
Needle Strike and Dazing Shot share the same damage, but their secondary effects are quite different and good. However, Needle Strike’s passive outperforms Dazing Shot.
Needle Strike boosts the next hit you do by 7%. So a 20%-100% ability like Binding Shot, for example, will do 21.4%-107% damage, and Snipe would be boosted from 125%-219% to 133.75 %-234.33% damage. It can also boost BOTH hits of Snapshot, so it will go from 200%-330% to 214%-353.1%. And if using multi hitting channelled abilities like Rapid Fire, the first two attacks will be boosted. That doesn’t stop there though if you follow up another ability after Snipe, that hit gets boosted as well, so you can follow up Needle Strike with Snipe and Snapshot to buff both hits within one Needle Strike, which is a huge DPS boost.
Dazing Shot, on the other hand, lowers the next attack’s accuracy by the targeted opponent by 10%. This makes it pretty nice for reducing a bit of damage and it certainly adds up, but it pales in comparison with Needle Strike.
Aside from that, the only difference worth noting is that with Shadow Tendrils, dual wield can sneak in an autoattack for free, lossless damage and potentially 2% adrenaline. This helps dual wield edge out even more against 2h.
Unload and Incendiary Shot are just like other ultimate abilities – not worth using. Skip out on them. However, Incendiary Shot can get very niche use with the Decimation bow’s special attack, Locate, where all of your attacks become AoE. That even applies to Incendiary Shot – in a huge cluster of enemies, multiple Incendiary Shots will hit the same target, hitting for an absolutely insane amount of damage.
Like Melee, dual wield and 2H can complement each other though. For example, you could Needle Strike, then when sneaking in that auto attack during Shadow Tendrils, you could switch to your 2h and sneak in a powerful auto attack inside your Shadow Tendrils, breaking the effective 12k damage cap and potentially hitting way more in a tick. Taking advantage of Needle Strike then switching to a higher-tiered 2H (Ascensions with Seren godbow) is extremely good DPS. Unlike Melee though, the DPS gap is not very high in comparison, so if you don’t bother switching you won’t be missing out much. Plus, without a Seren godbow, switching to 2h is extremely annoying due to needing to switch ammunition as well.
Magic
Outside of abilities, there’s a special feature of Magic, exclusive to itself alone: debuffs. Debuffs like Vulnerability, Stagger, and Enfeeble are extremely helpful in weakening bosses and helps immensely in tanking. Depending on if you’re using dual wield or 2H, your casting speed to apply debuffs will vary. If using dual wield, you can apply debuffs every 1.2 seconds, but if using a staff you can only apply one every 4.8 seconds. So if you need to apply a lot of debuffs, take dual wield. If not, it’s up to whatever other factors you have.
Concentrated Blast does three hits that do 15%-75%, 16.4%-82%, and 17.8%-89%, up to a total of 49.2%-246% damage over 3.6 seconds, so 147.6% average damage and 24.6% ability damage per tick. It also has a bonus effect in adding critical hit chances to your next hit, starting from 5% to as high as 15% with each hit added, assuming the hit landed. This stacks with all other sources of critical hit bonus additively, so if you have Biting 3 and used two hits of Concentrated Blast, you’ll have a 16% chance of a critical hit on your next hit. A cool thing is that this includes each critical hit chance working on the next Concentrated Blast hit. For example, the second hit will have a 5% bonus critical hit chance, while the next hit after, be it the third hit or another ability, will have a 10% critical hit chance. However, when using Concentrated Blast, you don’t let the third hit register – you use another ability right as the global cooldown finishes, so you do 31.4%-157% damage (94.2% average damage) over 1.8 seconds, for 31.4% ability damage per tick.
With this in mind, depending on the circumstances it can vastly out-DPS Sonic Wave, which does does 31.4%-157% damage, the same as the first two of Concentrated Blast’s hits. Instead of a critical hit bonus, Sonic Wave comes with a bonus perk in adding 6% hit chance to the next successful hit, should Sonic Wave itself hit. Sonic Wave’s accuracy boost works on both of Wild Magic’s hits, but for multi hitting channeled abilities like Asphyxiate, only the first attack will be boosted.
Comparing Concentrated Blast and Sonic Wave gets slightly more muddled compared to comparing Needle Strike with Dazing Shot. Because Sonic Wave’s bonus is useless with good accuracy, Concentrated Blast is way better for pure DPS due to the potential critical hits from all abilities that come after it and for the later Concentrated Blast hits itself. However, the value of Sonic Wave’s hit chance boost compared to Concentrated Blast’s critical hit boost gets better and better the lower your accuracy, and if you have below 94% hit chance, Sonic Wave beats out Concentrated Blast in terms of pure DPS, even accounting for Concentrated Blast’s higher damage on average.
With that said, Sonic Wave is better than Concentrated Blast in high-end PvM with lower accuracy. With a 6% hit chance bonus and more mobility (Concentrated Blast locks you in place while you use the ability), there’s less of a reason to DPS with dual wield Magic than with 2h. However, given very good accuracy, Concentrated Blast can easily top Sonic Wave’s capabilities, so use whatever is better in your situations.
Of course like Melee and Ranged, you can complement dual wield and 2h together. Like Ranged though, there’s not much to gain from switching. It’s easier by far, but still in a good rotation you have very little need for Concentrated Blast switching, doubly since under a good Sunshine rotation, you have little need to use it.
Conclusion
Melee
Personally I'd suggest to buy Drygore long swords if your on budget or working your way to Noxious Scythe.
Magic
Id suggest to use 2H if your more into a casual approach to PVM or want something more simpler so you don't have to cancel Concentrated blast.
Range
Like Magic Id suggest use 2H to keep things simpler if you don't wanna switch bolts and want a bit more range for places like Nex where having longer range helps
Overall you can't really go wrong all have their place just look at your situation and really look at your play style, the content you do and the budget you have.