Their returns are very different; it's tough to tell. Federer returns close to the baseline, Nadal stands extremely far back.
Nadal breaks his opponents service games more often, but I think that is more to do with him being better in the rallies once he actually gets the ball back into play - rather than being better than Federer at the actual return shot itself.
On both hardcourts (32.3% vs 31.7%) and grass courts (30.7% vs 27.7%), Federer has a higher winning percentage on first serve returns than Nadal. This suggests that the defining difference is that Federer has a better "reflex return" - that is, he can block a big serve back into play better. Nadal, on the other hand, has a higher winning percentage on second serve returns on all 3 surfaces - which suggests his advantage arises when points begin in a more neutral position and he can get into a rally.
I think their head-to-head records against big servers support my argument. Federer has a winning record of 87.5% against the top 10 players who hit most aces on tour (Karlovic, Opelka, Isner, Raonic, Kyrgios, Querrey, Anderson, Bublik, Copil, Lopez). Nadal's record against the exact same players is 81.1% - lower than his overall career winning percentage, which suggests that he dislikes playing big servers more than Roger.