Restaurant, unless we're talking about the Sterling Buffet in Las Vegas or something. Or a high quality Korean BBQ place where you cook your own meat. Those buffets bring the glutton out of me.
But Restaurants still win.
When you're going out to eat, you're looking for a dining experience. Not to stuff yourself with matter. The ambiance, the decor, the service, and food cooked by passionate people you don't get to taste everyday. That's what you should spend your money on. When I get invited to those cheap buffets, all I see are people stuffing themselves with chow mein or mash potatoes and gravy. Like really?