Red Faction: Armageddon or Portal 2?

I haven't played Red Faction: Armageddon, but I thought Portal 2 was extremely fun. It's especially great if you've played the first one, but it's great either way. I personally didn't find any major flaws with the game. You can play it single player or co-op, too, which is very cool. I would buy Portal over Red Faction.