I've done some local tests on revisions 10, 12 and 13b, my results are as follows, keep in mind this will vary with your usb drive.
Revision 10, caused some few games to occasionally and randomly freeze. Not a hard drive spindown issue.
Revision 13, ran all games well and had great loading times. Remember to use the right USB port. Has a dealbreaker bug for me though - you have to shut down and turn on the Wii to load a second game, as a loader will likely (always with USB Loader GX) freeze trying to load a second game.
Revision 12, ran all games well and had the same loading times as revision 10, about the same as DVD. Return to menu and start loader resulted in no glitches.
So for me it's Revision 12 all the way - best compatibility with games and no freeze issues on second run.
Of course, if you use USB instruments or peripherals in games, like microphones for Rock Band or Animal Crossing, they work best in revision 13. I play none of those games (or at least don't have those Rock Band peripherals) so 12 is the clear winner.