I have both the Beyerdynamic DT990 Pro and the Ultrasone PRO 900.
Here are my impressions:
-DT990 is more comfortable and lighter
-Pro900 has better bass and a flatter (perceived) overall frequency response but it can also get more edgy and piercing than the relatively well-behaved DT990.
-Soundstage is better on the DT990 but imaging is better on the Pro900. This is due to the S-Logic technology that has unfortunately been known to not work for a lot of people. Works for me though, and it gives a pretty good idea of the location of a sound.
-Pro900 tends to be more expensive unless you're willing to buy second-hand
-Pro900 is far easier to drive than the DT990
-Pro900 isolates from outside noise, while the DT990 doesn't (at all, its as if they weren't even there)
My final opinion? I don't really know...
DT990 would be better for long gaming sessions, where huge sound effects don't play that much of a role. For example, playing Skyrim or Dark Souls with the DT990 on my head was a quite pleasant experience (well, as pleasant as Dark Souls can be). They never got fatiguing and I basically forgot they were even there after 15 minutes or so.
The PRO900 is a bit more fatiguing and forward. I think it is far better suited for fast action games with lots of sound effects and a busy soundtrack. The extended low-end really works great with explosions and the superior imaging helps to locate enemies in FPS games better. The PRO900 is not as comfortable though and their consistently forward, detailed and edgy presentation might be too much for relaxing gaming sessions.