Here is my comparision:
Sampras 14 slams vs Federer 12 slams
Sampras 7 Wimbledons vs Federer 5 Wimbledons
Sampras 6 year end #1s vs Federer 4 year end #1s
Sampras 5 U.S Opens vs Federer 4 U.S Opens
Sampras no career slam vs Federer no career slam
Federer best year 3 slams wins vs Sampras best year 2 slam wins
Sampras at this point has the edge in almost every important department. Federer's French Open record is better, but people wont care much about that unless he wins the French. After all people dont bring up often that Lendl's Wimbledon record, and Borg's U.S Open record, are both better then Sampras's French Open record. In the end they are put in that same boat as far as their "weakest slam" since they didnt win it, when you are talking about players of that calibre that is all that really matters, and if Federer does not never win the French it would be all that matters regarding his French Open record as well. Furthermore, as noted before by those of us who followed the 90s with any interest, Sampras had far superior competition to Federer. However this argument only needs to be weighed further if Federer surpasses Pete's achievements, at this point Pete has achieved more then Federer has in almost every conceivable department. Roger has the edge of more dominant best years, that is an advantage in his corner, but that does not outweigh all the things he is still trailing in at this point.
I would like to touch on something else, I know I will be flamed for saying this but it is my opinion. While I concede Federer is definitely a better clay courter then Sampras, Sampras from 93-97 was still a darn good clay courter and better then people credit him for, while Federer while an extremely good clay courter still benefits somewhat from the lack of depth on clay today. However more importantly, and the main thing I see is the fact Federer is a better clay courter then Sampras, while a positive in one respect, actually leads to a knock against him in another respect. I say that since unlike Sampras he is good enough on clay to come close that many times at the French but keeps coming up short, I tend to think Sampras were as good a clay courter as Roger he would have won the French by now (in either time period, then or now). Yes if that time period where Pete hypothetically was equally good to Roger on clay had him playing today, that would mean Nadal also being better then him on clay to the same extent he is better then Roger on clay, but the gap between Nadal and Roger on clay (and thus in this hypothetical of Pete being as good as Roger on clay between Nadal and Pete) is not big enough to keep Sampras's determination and sheer internal drive from having won it atleast once by now. The gap between Nadal and Roger on clay ability wise isnt as big as made out to be, if it were Roger would not be able to take atleast a set off Nadal, and have chances to do more then that, every single best 3-of-5 match they have ever played on clay. Roger I think is just not comfortable when he isnt dominant or clearly better then his opponent and simply isnt willing to dig down enough, even when he is close enough to be in with a shot, and even with the potential huge implications at stake. I think Pete with such a major prize would not allow himself to not have won the French at some point vs Nadal with the same ability as Roger on clay, the gap is simply not big enough in clay court ability to keep him from gutting it out at some point.