The problem with Sampras was always Sampras. He was more talented than Federer but he didn't have the desire since he beat people so easily. He didn't work on his game like Federer or train meticulously. Feder has a more complete game than Sampras but when Sampras was on with his serve and volley, it was unstoppable. I also felt that Sampras had tougher competition because Agassi was in his prime and even old school vets like Connors and McEnroe made grand slam semi appearances in the early 90's. Tennis was a much bigger sport and thus attracted better athletes.
The other problem with Sampras is that he didn't have Tiger Woods to compete with either. Woods has motivated Federer to keep winning grand slams.
I still think Sampras could beat Federer if he devoted himself to the sport like today's athletes meaning he would train hard not only on his game but his body as well. I just don't see Sampras doing that. Samras just doesn't have that attitude. He is not going to update his racket or lose weight to gain speed. He wont' do yoga or train hard for 6 months on just his body to get in peak shape.
It's like an Emmitt Smith vs. Barry Sanders debate. Like Emmitt, Federer will get all the trophies but if Barry was 100% devoted to his sport the way Emmitt was, there is no way Emmitt wins the career rushing title. However, I do agree that offseason work ethics and practice off the field/court is part of the equation and an athlete shouldn't discounted because he worked hard off the field.