I listen to some trance. I would say the TH900 is much better suited for the job. HD800 is too neutral and polite to be as engaging as the TH900 which gives you 99% of the detail that HD800 gives but at the same time gives you some nice bass slam which HD800 can't give you.
So overall for trance and for any genre really where bass is important, TH900 gives you more of a engaging and "toe tapping" sound.
HD800 is still pretty nice though. For trance though, 9 times outta 10 I will reach for MY TH900 for some chair raving
Trance is one of the genres I'm not a fan of and I heard many times the 900 does very well in that area.
But I can tell you theat's not the only genre the 900 does well, it rocks like helll and I enjoy it a lot for classical or the vocal/jazzy stuff as well.