I haven't played either, but 2k7 is way better the nhl 07 IMO. The 2k games are just more in depth and skill-oriented. EA always has better graphics and is more fun for multiplayer, but that's only because 2k gameplay is so detailed and takes a while to learn.
I'm not really sure where I'm going with this since I haven't played either game, but if I had to guess, I'd think that 2k8 is the better game.