I own both the AD700x and Q701 so maybe I can speak to this. Evshrug and stillhart are both right, the differences between the two are moot. Both are in the upper echelon for competitive gaming. The only reason I "upgraded" from the AD700x was purely because I wanted to use the Q701s for music listening purposes as well. The AD700x is pure gaming bliss for a competitive FPS. If you go the q701 route you would also likely have to buy an amp so even though I would recommend the q701 over the ad700x, if it is for purely FPS gaming, you will be more than happy with the 700x.
As for build quality, I had zero issues with the AD700x. Yes there is a lot of plastic but it seems that the headphone will hold up just fine over time. My only gripe was that the wings on the headband would cause the earcups to slide down over time, causing constant readjustments mid game. This can be easily fixed by doing the "rubber band mod" and isn't too much of an issue. Other than that they are extremely comfortable. The Q's are as well, but a lot of (myself included) have an issue with the bumps on the headband. This is also an easy fix as you can buy a wrap on eBay, a baby seatbelt wrap (sounds strange but it works), or just mod your headband if you are brave enough.
TL;DR - AD700x is a great headphone and build quality shouldn't be too much of a concern for the price paid.