Correct me if im wrong,but Becker and Sampras never faced each other at the French Open.
Courier and Sampras played each other twice at the French Open,with both winning once.So i dont see how you can say that both players were obstacles for Pete.He was outdone by much lesser players at the French Open.He just couldnt pull it off on Clay.
Nadal on the other hand,is a true obstacle for Federer.
Federer can play very well on Clay,and Nadal is the only one stopping him from a much better French Open record.Sure,we all have personal opinions,but Federer is the more complete player compared to Sampras.