I don't know. I think that Sampras will always be the best of a era, while Nadal (despite not from the exact same generation) always has Federer in front of him. Nadal wasn't the absolute best for very long.
I think it'll be close, but Sampras will be better. 12 years between his first and last grandslam also makes him memorable.
However, to tennis historians, Nadal's clay records will always be looked at with amazement.