You can't really compare Sampras and Nadal for many reasons, Nadal will not win 14 grand slams, Sampras never won the french and Nadal dominated for 4 years, Nadal will not be winning 7 wimbledons or have more weeks at number 1 than him and their style of play is completely different.
I think most people compare Federer and Sampras because they are grass specialists. I think a better debate would be to compare Nadal and Bjorn Borg since as they both are clay specialists who have extended their success onto grass.