Sampras did have that point-ending running forehand. Made even Agassi pay for going into the open court. Every time. I have a feeling that Federer would've wanted that shot this year at Wimbledon.
But other than that, Feds' is the most versatile weapon. Punishing on any surface, from any part of the court, and during several different types of rallies.
Exactly Sampras' forehand on the run has been the best on the run forehand these past few decades. However, Roger's forehand is a lot more versatile and affords him different winning tactics with that wing. Sampras's proved to be consistent all career long, Rogers has been shaky at times these past two seasons and it's yet to be seen, so one has to wait and see after their careers are over who had the better forehand. I for the moment give the slight edge to Roger, but my gut feeling keeps telling me Sampras's had the consistency.