Sampras had to play against far more dangerous players on the surface
Sampras could serve himself to the title. And really - how many other all time greats did Sampras face in those finals? Nadal has won 5 of the 7 against Federer & Djokovic, one against Soderling - the only guy to have beaten him on best of 5. These aren't mugs - the former are incredible players who'd have FAR more claycourt titles if it wasn't for Nadal stopping them every time.
7 Wimbledons, 7 RGs - both are VERY impressive. Nadal takes it IMO as he has 7 in 8 appearances, vs 7 in 14 appearances for Sampras. Nadal's number may go down but he probably won't get to 14 appearances anyway so his % will always be higher.