Well Castle Crashers is 4 player co-op and hella fun. If you plan on frequently playing with your friends at home or online then DEFINITELY get that instead. Even if you plan on playing with just 1 friend you can each be on even playing fields.
If you want a single player game then Deathspank is MUCH more enjoyable. The co-op is ok, but your partner doesn't get the same awesome privileges as Deathspank. He's more like a tag-along. If you have a little brother to play with and he doesn't mind then you can definitely enjoy the co-op. Also the co-op is local only. No online play.