Mass Effect 2.
Were this any other situation I'd put my vote in for Square's game, much preferring their settings, if not their characterization and storytelling style. Not this time. I know what Final Fantasy XIII plays like. It's bad. Very, very bad. Basically: you go through a linear environment, fight semi-random battles, watch a cutscene, rinse, and repeat. You don't explore because there isn't anything to explore. You don't do sidequests because there aren't any. You don't interact with NPCs in any meaningful fashion, either; not even to purchase items. Instead, you buy 'em at save points.
It sounds like a sick joke (actually one of Yahtzee's sick jokes), but I absolutely assure you that it isn't. Please, if you have to buy one of these games, go with Mass Effect 2. Bioware's trying to improve the base systems and the quality of the overall product, and it's going to be a lot more stimulating.