Sampras has the better grass court game, though it's grass so who cares.
Sampras has a better 2nd serve which is the most important shot in grass court tennis. His first server is also better, 2nd most important shot, and holds up under pressure. His volleying is a great deal more superior to Federer, (3rd most important shot) who is not a great volleyer, complete different league. Federers slice return is better than Pete's and his movement left and right is better. Pete has the best smash in tennis which although in it's self is not important, it forces the opponent to go around or through the volleyer. Unsually offensive lobs are played at important points in a game so they aren't a highly played point but they are played at significant times.
Federers problems are his volleys, he volleys short nearly all the time and is not consistent. Short is good, it's either a winner or loser but playing short too often is scripted he needs a defensive deep volley or 3/4 court slice volley.
Grass courts obviously have changed, they've changed the grass type to a more resilient grass which makes the courts a "touch" slower. But it's the predictability of the manicured grass that makes the greatest impact. Sampras also played on slow grass, the first week at Wimbledon is slow, it's a green top, the ball bounces and is generally predicable, the difference is the 2nd week where in the 80's (go Mac), the court was a sand pit in the service area's and the center Tee and green on the sides. Any ball hitting within 1m (3') of the baseline and 2 to 4m (6' - 12') from center is anyone guess at how it will bounce. Hence grass court players don't let the ball bounce and hit compact shots. The courts have become more consistent with artificial lighting, changes in grass seed, originally the "tent" now the roof to control rain and allow artificial spectrum specific light. The courts weren't as bad in Pete's era as decade previously but it was no where near as good as current generation.
Anyone who's played competitive grass court tennis will know it's a different game and Wimbledon have gone out their way to make it more like a fast hardcourt surface to remain relevant. Grass court tennis is for another era, it was removed from USO & AO it's only the prestige of Wimbledon allowing it to hold on, otherwise it would go the way of carpet which was also a big tournament surface in the 70 & 80 with the masters and big indoor invitationals. Socially grass court tennis is dead, it's requires a court for every 8 competitive players in order to rotate courts every 6-8 weeks depending on usage and weather. You need a full time grounds keeper and equipment to maintain it. Grass court being expensive tends to be wealthy area's where the land values are high and resulted in a lot of clubs have converted to "synthetic" blasphemy, and using less land and pocketing the cash. The grass court competition is nearly non-existent in my country as there's not enough clubs for proper inter club competition rather it's just a few gentlemen club.
Wimbledon is now an anomaly surviving on the importance on the event rather than the sport. Pete Sampras was the better grass court player and was the tail end on the grass court relevance as a game rather an an event. Once the players like Edberg, Becker, Stich, Ivanišević, Rafter, Cash, Old McEnroe retired they weren't replaced with true serve volley grass court players, not because Wimbledon changed rather the whole circuit changed and the game it's self especially technology favoring base line.
