This link should help:

http://poser.smithmicro.com/poser9-poserpro2012/index.html?/#compare

Biggest downside to 2010 is no support for Weight Mapped characters...like WM v4, etc.

Well...SSS and more as well, LOL!

Actually, IMO, a sidegrade from P9 should be cheaper, cause you get less features and reason to upgrade....but it does not work that way.