How long do you play WoW for. I loved my Beyers, don't get me wrong, but I found them to be uncomfortable after a while. I play WoW for almost 6 hours a day. So to me, the comfort was most important. That's not to say they weren't, but after awhile, I felt I had to air myself a bit; probably advisable regardless of the headphone type though. I have an AD900 now, and I think the A900 might be similar, weight/comfort wise. And just like the above poster said, the A900 might be better comfort wise.
You might want to look into an open can if you just game WoW. Reason I got the AD series, is that I found I didn't like that in your head feeling, not sure if it's just the beyers, or maybe closed cans in general. That's why I didn't try the A900, went straight to the AD900. The AD900 has a beautiful airy wide soundstage, that I find suits WoW real well. Open cans are in general better for most kinds of music listening, IMO.
On the flip side open cans are not suited to other types of gaming, like FPS. I played Quake IV with the Beyers, and the positioning was second to none on them. And the 770's make love to explosions like no tomorrow. The AD series, probably has to be EQ'ed a bit to get that bass punch, which I'm not a fan off. And the explosions lacked that special punch to make it satisfying.
I know it's probably not what you want to hear, but you'll have to try them out yourself and see which one you prefer.. that's what the FS forums are for
.. buy, try buy try, sell... expensive hobby this.
I think you can't go wrong with either one, but I think the Beyer's have the greatest upgrade path potentional. You can get an amp later on if you don't get one right off. Also they can be modded to Darth Beyers, with a recable or have them woodied with screened covers. Supposed to take care of the recessed mids, take away the top end brightness, and a screened woodie might give a nice airy soundstage. Might try one of them out if they ever show up on the FS thread again.