I'll post what I PM'ed to a user that was wondering between the two as well.
Both the HD598 and the AD900 are two of the finest open headphones I've encountered. If I had to pick one right now, I'd take the HD598 due to superior comfort and preferred sound, and the AD900 if I was going for build, appearance, or design. I actually had bought the HD558 to compare to the AD900 and it actually caused the death of it. The HD558 added that missing bass that completed the spectrum. With how much I liked it, I knew I had to get the HD598, and boy did that improve upon the HD558 quite a bit. So, I guess you can say I find the HD598 superior to the AD900. It's smoother, more lush, warmer/enveloping, and has the missing bottom end from the AD900. The AD900 on the other hand is more resolving, has a bit more detail, is more clean and crisp, more treble energy, and also more upfront. Soundstage between the two is about the same. HD598 wins out on comfort, due to the AD900 driver touching your ear. HD598 is all plastic while the AD900 has magnesium and "higher" quality components. All in all, both are awesomely stellar, but with the HD598 running for $180 and the AD900 for $200, I think I'd take the HD598. Although, there is something about the AD900 that I really miss. Both are just fabulous though.