Farcry 2 is wayyy better.
Halo 3 has a predictable, shitty campaign. The whole Halo series sucked ass.
Farcry 2 has a pretty huge campaign (Takes a lot to complete, and is an open world gameplay experience.)
It has great online gameplay, with weapons that can be upgraded to different weapons, different types of explosives, etc. And the ranking up and class system is pretty neat.
Plus my favorite feature, is that you can create your own maps, and make them as little or big as you want, do whatever you want to them, and then either play them with your friends, or online with the world.
Either way, Farcry 2's online, campaign, and support massively tops that of Halo's.
@Answerer # 1, graphics don't mean anything if it has shitty gameplay.