All surfaces considered, it's very clear that Borg is superior to Nadal imo
As for clay, both were so superior to the others that yes, despite better figures for Nadal, I personally don't think it's that "obvious" that Nadal is better than Borg.
They just had a different attitude towards clay and towards tennis in general : Nadal plays everything like an oger, typical of our era where people are so much concerned about records, Borg didn't put so much energy there, was more "dilettante" in a way. Often preferred playing carpet tournaments than clay ones.
Imo, both were "clay monsters", including physical monsters, and that's all.