U're comparing hit items vs no-hit items. so if u have hit cap - staff, if u don't than mh+OH.
In case when there won't be any hit in 2 options between staff and mh+oh, mh+oh wins cause of higher total SP number.
Also it depends from spec: destro/hybrid spirit efficiency is pretty low, so even lower ilvl crit+haste items can beat spirit ones.
Need more info for this questions, at least armory link.
As for BiS: mh+oh (bloodsurge +spindle/sundial).
For the most part i played demonology, now that im over 5k gs im gonna switch to affli, unless the raid demands demo lock.
Point is i have the set of dagger and OH...but somehow i dont think staffs are gonna be worse to preform on warlocks, plus i really like staffs nad looks of the locks with staff.
In affli im hiting hard cap, with demo i hit 3% less(suppresion talent) but if i remove the dagger and OH i lose the hit raitng, meaning i would have to stay in Affli spec to maintain the hit cap
Use staff in affliction, and mh+oh id demo than. 90 hit is almost 3% that u got from talent - ez way.
But tbh, i recomend u think about your future gear improvements. Cause losing SP from switching theese weapons to trade for hit is bad idea. Just Cap 17% for affli (14% from gear + talent), and use 14% (from gear) in demo as far as u're planing it as secondary spec for raid (and in majority of theese raids will be sp/boomie with hit buff). And when u will get some unset parts to trade hit to sp, or sp to hit, use them.
Better build your comp with mh+oh and use hit gems if needed. (staff sp+2 sp gems+81 sp chant + 0.6 spirit = 764 total sp; mh+oh sp + 63 ench + 1 sp gem + 0.6 spirit = 805 total sp, that is 41 more than staff)
I dont have a staff, just wanted someone with a bit more math brain to explain why so many ppl just ignore staffs that are awesome in so many points :D
Im a sucker for everything that looks cool and thats the case with lock and staffs. Daggers just dont have that "feel" on warlocks from visual point of view...
Thats why i wanted some advices on how much is a difference in using one over the other
Yeah, there are much more cool staffs than main hand weps.
But main reason to not use it, cause in case of 2 options with same ilvl and same secondary stats (crit, haste, spirit) mh+oh always has more SP.