http://tt.tennis-warehouse.com/showthread.php?t=421939 inspired me. Both often defensive shots. To my perceptions anyway, Federer used his slightly more, especially as an offensive shot. Sampras in his early years had a very attractive topspin backhand. In contrast, his slice floated too much. Indoor accentuates this. Maybe this is why he did not experience early success at Wimbledon. Sampras talks about 'firming his backhand slice up' over the years. It certainly became a valuable part of his game. I don't know whether Federer had similar problems in the early part of his career. As you see though, the backhand slice is not such a crucial shot, not on any court, though variety is nice. Federer's slice seems to have more spin than Sampras', however.