If we want to just choose the Ultimate indoor title which is the YEC... Then its not difficult at all to choose: Nadal won some other indoor titles so that helps I believe but.
Sampras for sure. He won the Davis Cup on slow as molasses clay (almost single handidly beating the Russians) won Rome, and beat some big names at the French Open with a few QF appearances and a SF appearance.
If not for his Blood Condition ( something of which he had no control over either)and the death of his coach I have NO DOUBT he would have won the French as well.. Or if he played in a crap clay era too of course.. Unfortauntely, the 90s was some of best on clay when it came to depth.
And Sampras wasn't a clay mug ( in his prime).. No he wasn't the best but its not like he has fucking Roddick's resume on clay for god sakes. He has some big clay titles and beat just about all the best the 90s had to offer on clay (Bruguera, Courier, Agassi, Muster etc)
No, Sampras was very bad on clay. Blood condition or not his backhand was absolutely disgusting on clay and so was his movement, proper tactics was nonexistent. We all watched him play, some things are difficult to forget. In this clay era that you call crap era he would never win a set against Federer, Nadal and Djokovic. He never got to play the better part of the 90s when Guga came along and became a dominant force competing with Corretja, Moya and Ferrero. But I do agree he had the slightest chanse to win the 96 RG with better conditions, Kafelnikov was his bitch and old Stich is very beatable in a RG final.
Anyway, I think Sampras might just have been a little bit better than indoor Nadal on clay. Their achievments are almost identical.
But I do belive Nadal would get to a SF in a best of 5 indoor grand slam, he can beat anyone but the best and a couple of specialists (Davydenko, Nalbandian).