I'm a big fan of both I've played all the assassins creeds, and they're fun, but repetitive. Sneak here, kill this guy, escape. Excellent parkour, though. Fallout has a ton of different ways to kill things, and an abundance of locations and people to meet. (Not to mention fallout new Vegas has 65,000 lines of dialogue) but, after a certain point, you become so OP it loses its fun. And you have almost no backup in fallout 3, aside from a few followers, who you have to have a certain karmic requirement to get. Honestly, both are great, and I regularly play both, but fallout has to win this one for me