If there going to do it right, the difference will be quite significant. But in reality Crysis 2 on the PS3, isn't going to look even close to Killzone 2 which serves as proof that there not 'maxing the system'.
Maxing the system means nothing, really. They probably ARE using all of the processing power, effectively 'maxing' the system, but if there code isn't optimised properly there not going to be using it in an efficient manner and in turn it won't be producing the best results.
As a multiplatform developer both versions will be the same, in the interest of parity. I wouldn't be surprised if the PS3 version is slightly worse just because PS3 development is generally more difficult, and I'm not sure how lazy a developer Cryek are.
In the end, if you're a PS3 owner, Crysis 2 isn't going to be a big deal, and isn't going to be raising any graphical bars for the system. Judging by the god-awful console footage we saw at GDC anyway.