Well I don't actually have any CODs on PS3 but I am thinking MW3 it seems fun to me but I will always also like RPGs. (I have Black ops, Modern Warfare 2 and Modern Warfare Reflex although they are all on Wii so they well.... are shit)
"Always Extreme at heart."
I don't check the forums or go in-game anymore, I have quit.
to be honest: In your case I'd rather buy mw2 instead of mw3, because mw3 is completely the same game with a decent SP story and the same MP as MW2 (except that every online map is a post-apocalyptical version of a shitty map ).
Oh and btw: it costs ~40€ more