Purely my opinion, but I reckon Sampras at his best ('99?) would have beaten Federer in 2003, 2009, and 2012. Federer at his best ('05-'07) would have beaten Sampras in 1993 and 2000. I also reckon Federer's best was slightly better than Sampras', but that Pete's average standard during his reign was slightly higher than Roger's.
I couldn't pick between them, not on grass. I'm quite happy to say they're as good as each other and leave it at that.