In a previous post, I had 2004 Federer and 2010 Rafa splitting, with Fed winning on the fast USO surface and Nadal winning on the slower surface. I believe that I was extremely generous to Nadal. I think that I might have been overly-generous to Nadal. But his 2010 USO performance was insane. I may end up switching that to 2-0 in favor of Federer. Federer's hard court stats simply blow Nadal's out of the water. And this is despite the fact that Fed's stats after the age of 30 aren't close to what peak-Fed did
Here are their USO stats for their careers:
Federer
78-11, .876 overall
16-6, .727 vs top 10
5 titles, 7 finals, 10 semis
Nadal
43-9, .826 overall
5-4, .556 vs top 10
3 titles, 4 finals, 6 semis
The USO is a small sample size of hard court play. If we look at their overall hard court records, then the gap widens substantially.
Most people have forgotten that Federer owns the longest winning streak on hard courts, which is 56 straight. That record is unfathomable. Even in his old age, Federer is a beast on hard courts. Consider how unbeatable Djokovic was on hard courts in 2015. That year, Djoker lost the Cinci final to Federer. He then reeled off an impressive 24 consecutive hard court victories before having it snapped by the 34 year Federer. As as matter of fact, old Fed went 3-3 vs Djokovic on hard courts in 2015. That is seems impossible for an old man. Djokovic against the rest of the field went 56-2 on hard courts in 2015. Federer is a beast on hard courts, even as a dinosaur. And lastly, Federer from August of 2004 to February of 2007 went 137-3 on hard courts during that 2 1/2 year span. This is perhaps the most ridiculous run of dominance in history, outside of Nadal on clay.
Edit: That's 5 titles, 7 finals, and 10 semis for Fed at the USO