I actually can't even see why this is a debate.
Splitting apart 2 ways of distinguishing whose game is better.
Overall Tennis Ability
and
Overall Tennis Achievements
Overall Tennis Talent
In terms of tennis ability, i don't think there is much @ all separating Federer and Sampras serving wise, yes Sampras won many more cheaper points using his serve and served a lot more aces with it but the crucial factor many people always miss is the standard of returners in today's game demolishes the standard of returners in last decades. We also need to take in to consideration the fact grass was a slightly faster surface back then, hence the overall greater effectiveness in Sampras's serving.
With all this being Sampras still edges it due to the ability to bang down second serve aces.
http://www.youtube.com/watch?v=tt-vdBYeU3A - As highlighted so brightly in this video.
A quote taken from Wikipedia
He was known for producing aces on critical points, even with his second serves.[39][40] He had an accurate and powerful first serve, one of the best of all time;[41] His second serve was nearly as powerful as his first, possibly his most dangerous weapon. He had great disguise on both his first and second serves.
From a groundstroke’s view, Federer absolutely beats Sampras to the wall, the Federer's forehand is probably the greatest shot in the history of the game and anyone that questions that obviously knows nothing about the game.
Sampras couldn't hold his own from the back of the court, he had World class groundies, don't get me wrong, but they weren’t consistently and penetratingly devastating enough.
From a volleying perspective, due to the nature of the game today i don't feel Federer has had to master the shot as well as he potentially could have done. Many will say Sampras's volley was the better of the two, i am one that cannot decide, it is simply too close to call.
Versatility and stamina both go to Federer and for obvious reasons also stated above.
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Overall Tennis Achievements
Sampras still holds more Wimbledon crowns to his name than Federer, although I cannot foresee how Federer will not pass level or pass that mark with a minimum of 5 years still left in his career.
As has been well documented by many on this board and around the tennis fan community; Sampras never won the @ the French, Federer has, as well as this he has won French and Wimbledon titles back to back.
Federer has 5 straight Wimbledon and US Open Grandslams, given the field is easier on grass, i'm sure many will agree that is definitely not the case on hard.
Sampras took his grandslam tally to 14 in 2002, aged 33? Federer is on 15 and still going fresh, with the real possibility of now making it to the all amazing landmark of 20.
The only achievement Sampras has which Federer doesn't, to note, is having won a grandslam in his 30s and that is not Federer's fault.
The final verdict is Federer has Sampras comprehensively beaten in all departments. Prior to 2009, those departments were unfullfillied, however now, the answer is clear.
Federer is the greatest player of the last 2 generations and he probably will be the greatest player of our living one as well.