Neither headphone gets a lot of keystrokes here, but both are highly regarded - the AD900 is generally regarded as a better choice for gaming. I havent heard the A900, but my time with the A500 convinced me that the first few explosions in a shooter might be fantastic, but it soon gets tiring. For music, some complain that the AD900 doesnt have enough bass impact for them, but those who perservere are soon won over by the overall balance of these headphones. I dont have your budget, but my opinion is that its worth the extra to get the open phones.

estreeter

I don't know about the AD900 vs. A900, but in AD700 vs. A700 the open design sounds a lot better. The biggest difference is the sound stage (which is rather important in gaming). Regardless, if you want a lot of bass, neither the AD900 nor the A900 will do it for you.

I've got A900s and AD1000s.  I generally prefer the ADs, unless I want to block communication with my wife (outside noise is blocked a lot easier with the A900s) or listening to bass heavy music. 

I used to have the A900 (little brother broke them :( :( ). I never the felt that the bass was especially lacking. However wearing them for extended periods may get uncomfortable, as the heat will build up. You'll also need to replace the cheap pleather pads on a regular basis. However the closed design will prevent you from annoying your entourage if you're gaming where other people do stuff.

I'm also currently on the lookout for new phones (AD900, AD2000 or W1000x, haven't decided yet) so I am interested in what others will reply in the "where to get them" department.