I bought NHL 2K6 for the Xbox because it has the auto roster updates through xbox live. Which, by the way, I found out won't even be available until October. All in all the game looks like it was severly rushed. The controls are strange, the online play sucks, and the menus and "slider" options are out of the world hard to use and get used to. After I bought it I wondered if I overpayed for a $19.99 video game.
On top of all that ESPN is no longer affiliated with it so it looks plain as can be. It's actually kind of sad to play. I hope it's better when the roster updates come out.
NHL 2k6 is all about gameplay. The polish isnt that great, and the rosters are Aug 1st (you can fix them with about an hour). Losing the ESPN thing hurt. Still, with a little slider tweaking, the best representation of real hockey that you can get.
NHL2006 is also good. Looks great. Plays fun. Not necessarily realistic (you'll routinely have 50-60 shots on goal). Not very challenging, as theres not much defense. Fun game though. Lots of bells and whistles. Looks alot better than 2k6.
I personally played 2K6 more. Fixed the rosters.
It depends on your style of play. If you just want some quick hockey action, or play primarily against other people, NHL2006 is probably the way to go. If you play alot, like in franchise mode or, like me, want it as realistic as possible, go with NHL2k6.