if you don't play online ,,then i would suggest RE5.
(this comes from a KZ2 fan)
KZ2 is awesome but 60% of the fun is online and unlocking new abilities.KZ2 campaign is a bit longer than RE5 campaign but I liked RE5 campaign a little bit more.(controls are better in KZ2 though)
I would honestly get both.2 excellent games man.
my ratings: KZ2:9.7/10 RE5:9.2/10