Depends.
If you are a fan of deep online play then go for Killzone 2.
If you are a fan of co-op go for RE5.
Killzone 2 campaign lasted me 12 hours when I played it.
Not sure on RE5 length but it should at least be similar.
I am pretty sure though that KZ2 has much more repay value, so it may end being better bang for your buck if FPS is your kind of thing.
By the way, are you a Resident Evil fan? If so then that should make the choice simple.
It is all a matter of preference. For me I enjoyed Killzone 2's single player campaign a lot more than it's multiplayer. I thought it was damn good.
Edited.
iPhone = Great gaming device. Don't agree? Who cares, because you're wrong.
Currently playing:
Final Fantasy VI (iOS), Final Fantasy: Record Keeper (iOS) & Dragon Quest V (iOS)
