looking at the votes so far, it will be the Momentum and HD700. i was already planning on the Momentum for one of them.
after that, i might consider something like the HD600 and HD650 together, so people who have neither can hear the difference for themselves and decide which one they like better or if the HD650 is worth $100 more
I would really like the HD600/HD650 one.
I like this option too: it would allow a fair A/B comparison between the two.