I don't think it's a question anyone can really help you with, mate. I'd pick Gears of War 3, myself, as I view it as the best of the series and a damn good multiplayer game. of course, if you're Silver then you'll have to settle for playing against Bots
and on your own in Horde and Beast.
That would make Forza 4 better in sheer content terms as it has an extensive singleplayer career and multiple options to keep you playing.