In fact, surfaces and schedules are so much different today than in Borgīs time that the only common thing both have is their terrific top spin and having the two best ever records at the French Open.While I think both are the two greatest clay courters ever, for the rest of the surfaces it is quite different.
In Borgīs time, grass was extremely fast, making it amazing how he could switch in15 days from the ultraslow RG clay to the Wimbledon turf...and winning three times in a row while doing soNadalīs only won Wimbleodn twice, and the grass court is now like clay, just a bit faster.It is no possible to compare but, of course, while Borg is one of the 4 best ever on grass ( Laver,Sampras,Federer and maybe Tilden would join him), Nadal is like Connors or Edberg: he only won 2 titles, not a big deal.Not even fifth or sixth tier on grass ( even his clay court rgass)
In Borgīs time fast indoor carpet was as important as hard court is today.In reverseal, hard court was a secondary surface in Borgīs era and carpet has become the same in Nadalīs era...Nadal won 2 majors ( 3 if we add OG 200
on hard and none on carpet, Borg took three indoor majors and no major title on hard ( in spite of reaching 3 finals at the USO)
So, it looks pretty even here.
As a conclusion, Borg fared much much above Nadal on fast courts, and Nadal has a better record on slow courts.But Borg , retiring at 25, has a total of 14 majors and Nadal, AT 26, has a total of 12.
Thatīs the figures, rest is subjective