The main problem to consider when comparing Wlk3 (RC and FF being the best R10 “capstones”) and Wlk2/Shadow2 is whether you find Demon Scratch reliable. If we compare the classes solely by numbers and counting the skills that are unique to either build, we have the following:
Warlock 2/Shadowmancer2
Shadow Thorn 10 (with attribute) + Drain 5
3106% x 2 (attribute) x 1.5 (Drain lv. 5) = 9318% with 4 Overheats
Shadow Conjuration 10 (with attribute) + Drain 5
1589% x 2 (attribute) x 1.5 (Drain lv. 5) = 4676% with 2 Overheats and AoE no target limits
Notes: The attribute adds a DoT which applies 10% of Conjuration’s base damage per second and has a duration equal to the attribute’s level.
Mastema 10 + Drain 5
1717% x 1.5 (lvl 5 Drain) = 3863% no Overheats and no target limits
Notes: Gives a reliable (2x) Holy Debuff
while on the other hand, we have my personal build
Warlock 3/Featherfoot 1
Demon Scratch + Drain 5 + Curse (From FF)
2976% x 1.5 (Drain lv.5) x 1.5 (Curse Debuff) = 6696% 2 Overheats and AoE with no target limits
Notes: Behaves uniquely as it applies multiple hits as a function of distance travelled. You can get anywhere between 1 to 5 hits depending on the size of the target. Abbetted further with assistance from a Falconer
Mastema 15 + Drain 5 + Curse
2976% x 1.5 (Drain lv.5) x 1.5 (Curse Debuff) = 4574% no Overheats and AoE with no target limits
Notes: Gives a reliable (2x) Holy Debuff
Note that I didn’t specify the bonus from Mastema: Phantom Pain since I don’t know how it works.
Now that we have the facts out of the way, I’d say that either is actually viable but the main difference between them is utility and SP efficiency. Despite being promised that Shadowmancer skill SP would be “tweaked” I still found it quite expensive to use and given that the lack of AoE was somewhat troubling. To be fair, though, it is quite powerful. Using all four overheats gives you 36k% on a single target which is great all things considered.
Demon Scratch, on the other hand, can be equally if not more powerful depending on the situation. If the boss is particularly large–they usually are–or have a Falconer using Aiming, positioning yourself just right can help you pull off all five hits on two overheats maxing out damage at ~60k. However, that depends on how well you aim.
To be sure, you get more of the same from Warlock 3 and I’d pick Shadow 2 just for Shadow pool, Hallucination and Shadow Fettter. The first two being some of the best panic button skills (don’t quote me on this, but Shadow Pool seeems to remove all aggro from yourself acting as a very inefficient Fade unique to Shadowmancer) while Fetter is only one of two CC skills that work on bosses (the other one being Swap)
My Opinion: Go for Wlk2/Shadow2 there’s a lot to admire, but if the damage is not to your liking, you can try Wlk3 Featherfoot.