I am not sure how to answer as I am in the same boat as you, and am looking forward to reviews to determine which to get.
There is NBA 2K8 which looks really cool with the player reactions to good and bad times on the court, mascots, fans, and players all cheering when one player makes a shot looks really cool. More lifelike, less robotic.
There is also NBA Live 08, which from the demo has a lot of cool online options, and the best graphics of any NBA game in my opinion.
Then there is NBA 08 from Sony which is in 1080p at 60fps, but from the demo- it is hard to tell, it really looked very un-detailed,(kind of left me disappointed) Also the game's menu had punk music-which is not basketball appropriate imo. Supposedly this game has really cool options allowing you to control 2 players at once, and some innovative six-axis controls, as well as cool unlockables and online stuff similar to Live 08.
Without the reviews, I am leaning towards NBA2K8 because of how fun and lifelike it looks. they are moving fluidly- doing illegal moves, signature spins, un-necessary flash and celebrations- seems like a winner to me!