Just Cause 2 is fun if you want to blow stuff up in the countryside or w/e
Saints Row 2 is fun if you want to blow up stuff in the city.
Just Cause 2 is singleplayer, and is more about chaos and destruction (which is always fun).
Saints Row 2 is multiplayer/co-op, and is about doing crazy or silly stuff (mixed with some slightly serious stuff). It's like GTAIV, but more chaotic.