As far as Nadal/Borg, the AO makes the comparison difficult, because Nadal has gotten to play in a time period in which attending the AO is no cost to him at all (doesn't cut into his off-season) and carries every incentive to attend it and to win it (ranking points, prestige, prize money, competition, etc.). That is very different from Borg's time when the AO was all cost and no benefit. If you played it you would lose much of your off-season, and even if you won it, some of the same modern fans who would fault you for skipping it would then fault you anyway for winning a Mickey Mouse tournament. That's an unwinnable situation. The AO in that era came with a heavy cost and little benefit.
You could either adjust the comparison for the AO by speculating, or make no adjustment at all and simply compare Nadal and Borg according to their titles as they stand.
Either way Borg comes out with a lead.
If you drop Nadal's AO by speculating that he doesn't play a tournament with all cost and no benefit, Borg is leading in Slams by 11-10. Or if you speculate that Borg would have played the AO if it had been a Slam on par with the other majors, Nadal would stay at 11 Slams and Borg would have whatever number of AO's you picture him winning back then. He would have had a good chance to win at least a couple, but just one AO victory for him in the late 70s and early 80s would be enough to put him past Nadal.
The second way is to do without speculating at all. Leave both players in the conditions and circumstances that they played in, and just count the titles. Borg and Nadal are then tied at 11 Slams apiece. But Borg has two year-end championships, to Nadal's none. Borg also has far more tournament victories overall: 64 to 50 if you use the ATP's numbers. Other counts have Borg at 77 victories (see Wikipedia), and he's over 100 if all his titles are counted.
Of course you could say that Borg, due to circumstances in his era, got to rack up titles in a way that Nadal, due to current circumstances, could never do. But if you make that sort of adjustment then some kind of adjustment has to be done for the different circumstances regarding the AO. To do one without the other is an unfair comparison.