CALLING ALL GAMERS!! GTA IV vs Just Cause 2?!?! Which should I choose?

I know both are open world, an you don't have to follow the storyline if you don't want to. Are there uninhabited islands on either where I can go do whatever I want an not be bothered by anyone else? And most importantly can I either buy or steal like a house or property where I can stash copters on the roof and vehicles inside, like a regular house or a warehouse? With the helicopters on the roof so I can escape if I need to by air.just somewhere that's MINE. That I can barricade myself inside. Which game is better?!?! Which "world" is bigger help!!