Both games are going to be out soon in stores. NHL '08 will make it's debut on September 11th while NHL 2k8 will also be out in the fall.

NHL '08 has added:
AHL Teams
Improved Skill Stick

2k8 has added:
Pro Stick (similar to Improve Skill Stick). Basically, you can move the puck anywhere using the right stick and then shoot with the bumper.
New graphics engine (looks great)
New faceoffs (looks great)

Honestly, I may just get 2k8. Don't get me wrong '08 looks like a fun game and I'll probably rent it. It's just that NHL 2k's USED to be the best game to get. IMO, 2K's at least provide a more realistic hockey experience. And 2k8 has the franchise mode which is getting me really excited.

EA hasn't always had the great hockey series that they are starting up now.