I own 2K6 and downloaded the Live demo. Live looks better overall, but 2K6 plays *much* better. At first I didn't really notice it, but after playing 2K6 for 5 hours and then trying the Live demo, Live's gameplay really does suck. The player control is nowhere near as good as 2K6.
If you are buying for graphics alone, then Live delivers. For fun gameplay, 2K6 all the way.