Depends on what you want. The Q701 is more balanced, and a general all rounder. The DT990 specializes in fun, with great bass and helluva lot of sparkle in the treble. The Beyer equivalent of the Q701 is the DT880.
If you want find yourself playing more single player games, the DT990 is it. If you're more competitive in games like FPS, the Q701 is it.
I personally prefer the DT990, but since I have the D7000 (which would do the same tasks but better), I have no need for it. The Q701 is my complimentary can for all other uses.
if I were to put them in an overall tier list (for all uses) ...
D7000
HE-4
DT990
DT880 = Q701 (I especially like that the Q701's treble isn't harsh, but still sparkly enough).
As my personal faves, above all the rest I've owned. Again, that's for GENERAL use, not just for gaming, hence why the K701, HD598, and PC360 aren't on there, which I don't like for anything outside of gaming competitively.