There's a whole hell of a lot of romantic misremembering when it comes to comparing tennis players of the past to current ones. Even with the video available of Sampras you have to consider the vast, vast majority are of him playing his best - not his average level. As to whether he or Sampras is the better athlete you have to start with one question: what does "athletic" mean? It means slightly or greatly different things to almost everyone which renders most comparisons pointless. Notably, the fact Sampras did, on occasion, do cool slam-dunk overheads is a small and borderline irrelevant example with which to really demonstrate him as being a better athlete than Federer.