USB Overdrive definitely. I find that you can get much better feel to the mouse when adjusting the acceleration settings than with Steermouse. While Steermouse has better mouse acceleration than Apple's drivers, USB Overdrive seems to do it right even better.
The only problem is that USB Overdrive isn't updated all that often. Not that it needs to be, but right now I'm using Steermouse because USB Overdrive doesn't support the Mighty Mouse.
Featureswise USB Overdrive also seems to be ahead at least for regular mice, we'll see how it compares when using the Magic Mouse. Steermouse also doesn't seem to work with BetterTouchTool, which is a shame since it has far more extensive mouse gestures support.