I might end up getting Farcry 2 but I'm not sure why I would want to get it over Crysis Warhead.
Well, Far Cry 2 is more of an open world where you go where you like getting various side missions, etc. It's sort of RPG-ish in that sense (you also get 'money' - really blood diamonds, to buy stuff as far as I understand). Warhead/Crysis are single path story driven shooters, pretty much.