Federer is a better player than Sampras as he simply doesn't have a weak point, or surface for that matter. Sampras relied heavily on a booming serve but didn't have the craft or finesse to win the French Open, the grand slam played on clay.
Federer on the other hand has so many shots so that even when a couple of them aren't working 100% his game won't collapse. He may not go onto win as many grand slams as Sampras as Sampras was probably the best mentally atuned player the world has ever seen. Federer may get bored with tennis by the time he's 26 and decide to chuck it. I've heard that he's a pretty nifty footballer too.