Suit, koreans, aliens, special forces, voice acting, story, weapons, destructable huts, physics much more interactive.
The game does a heck of a lot more over Far Cry
I'll be honest, not played through crysis yet but cod 4 single player was meh.
IF you only want to play mp, then cod 4 is your answer I think, if you only want single player then crysis. If you want both then flip a coin...maybe crysis.