I mostly play online team play, so take that into account.
To me, 10 is better than 09. There are some lacking areas and glitches in both, but a few things make 10 better.
The auto-sub thing is annoying but it keeps people from running extremely cheesy lineups in team play.
The interior defense is much better. Almost too good, but it works out. In 09 you could easily get to the rim with anyone and just wiggle a bit then finish. In 10, there are cheesy work arounds like the spin move or the hard-cross that allow you to get dunks that the game wouldn't let you get otherwise, but overall it's really good. Again, sometimes they miss dunks that they should make and lay ups that they should make, but overall I think that less is better.
Contact is better. They made it so that a pick is a pick and doesn't get you stuck on people. They made it so that you can push people around and really manipulate where they go. so, you can start sealing people for rebounds before they shoot. Or you can push someone from one side of the rim to the other and seal them off while a player is driving (something that happens in the NBA.)
The Auto-block and Auto-steal animations work well, too.
Oh, and one of my favorite parts of 09 was taken out, but again, for the better. I miss the full court oop! I remember the joke between me and all of my club members was "oop it" because whenever someone took a bad shot, we'd be out. And the pg would just launch it and it would be a perfect oop 80% of the time.
All in all, I was a lot better at 09 than I was at 10, and maybe that factors into why I believe that 10 was better. 10 made it a lot harder and made defense much more of a factor.
Edit: Oh yeah, and they took out the go-to moves... or at least made them so they weren't so freaking money.
-Smak