I have played Live extensively over the past week or so and just got 2K10 today.
The FIRST impression battle goes to NBA Live.
Now, that doesn't mean much as I have played a grand total of 2 games of 2K10 but on the 360 the frame rate is SHIT. Total shit. I can get it to play OK thanks to a tip over at OS about zooming out in Broadcast view but I HATE playing with the camera that far out. Argh.
As I said before Live isn't perfect but so far I'm having more fun with it than I am 2K. I may need to edit sliders for 2K because out of the box there is no defense being played at all by me or the CPU.