the quick & dirty: K550 definitely had better sound stage & clarity & crispier/clear treble. Also had better bass extension down into the deep low frequencies. They could actually give a solid rumble when the soundtrack called for it, but I can see how some people may be disappointed w/ the bass quantity (being slightly basshead myself heh). I really loved the AKG bass though. I thought it was very detailed, textured with great tight bass notes. I never noticed the treble sounding metallic or silibant. Everything just sounded very clear & crisp, you can hear all the details that were covered up by more colored headphones.
The Momentums also had very good sound quality (clarity, detail). But nothing really stood out to me the way things stood out for me on the K550s. The Momentums do have a bass quantity boost compared to the AKGs.
However, both my brother & I agreed that the K550 had better overall sound quality. My brother actually noticed the bass boost on the Momentums & he has absolutely no audiophile/headphone background except for Bose Quiet Comforts lol. He said he thought that the Momentums were geared more towards a fun/enjoyment vs the K550 more clear for professional listening. He also brought up the discomfort issue with the Momentum. We had both for about 2 weeks and I ended hogging the K550 most of the time hahaha. It's been a while since I've direct compared em, so I can't really get too much more detail about everything else as I don't remember, but I remember strongly preferring the K550s.