This'll complicate matters regards your choice:
PS3 Mass Effect 2 uses ME3 engine
"We actually created the engine for Mass Effect 3 and used that to make Mass Effect 2 PS3. So we took the content, the story and all of the other assets that made up Mass Effect 2 and we put it into the Mass Effect 3 engine.
It also includes DLC content, so it sounds like good value.
And it comes on only one disc ...
Seriously, I'm glad it comes with a demo. I've been intrigued but not quite convinced by this game yet (only briefly tried the PC demo on a crappy PC). Will be good to see how the ME3 engine runs at least.
EDIT: doesn't who much, but here's a side-by-side:
http://uk.ign.com/videos/2010/12/16/mass-effect-2-touring-the-normandy
http://uk.ps3.ign.com/articles/114/1140265p1.html