It's amazes me the gizo says Sampras was better than Borg on three of four surfaces when Borg crushes Sampras in lifetime winning percentage by 84% to 77%. The 84% is from his actual lifetime record. Seven percentage points is huge and Borg won 106 tournaments to Sampras' 64 in a much shorter career.
Well which surfaces out of grass, indoors/carpet and hard courts would you consider Borg superior to Sampras on.
On grass Sampras did win the superbowl of tennis, Wimbledon, 2 more times than Borg which is a significant margin. I would argue that Borg did beat better players to win his Wimbledon titles than Sampras did, in Borg's case McEnroe, Connors, Nastase, Tanner, Gerulaitis etc, in Sampras's case, Agassi, a post-peak but still strong on grass Becker, Ivanisevic, Rafter etc. Still 2 extra Wimbledon titles cannot be underestimated, and having watched both players a lot I would take a peak Sampras on grass over a peak Borg.
Indoors, Borg played in an era where there were two huge indoor tournaments that were almost on a par with the majors, the WCT Finals and Masters. Sampras didn't have that same luxury (the Grand Slam Cup which he won twice anyway really didn't compare to the prestigious Dallas event). Still he won 5 Masters titles in 9 years, while Borg won a combined total of 3 Masters/WCT Finals titles during his career. Borg did beat Connors, Tanner, McEnroe and Gerulaiti to win his 1979 Masters title, and McEnroe, Connors and Lendl to win his 1980 title, so again incredibly strong competition. Still he lost 3 out of his 4 finals in Dallas. Sampras did hold the joint record for most masters/yec titles with Lendl before Federer came along, which has to carry some weight. There was a greater percentage of indoor tournaments on the tour in Borg's era than Sampras's (when outdoor hard court events had now become much more common). Overall I would also rate a peak Sampras over a peak Borg indoors (on carpet) as well.
Obviously Borg hardly ever played on hard courts that much during his career. Sampras played much more on clay than Borg did on hard courts, and still didn't achieve anywhere near as much on his weakest surface. He had some very good quality tournament wins on hard such as the 1979 Canadian Open which he won without dropping a set, drubbing McEnroe in the final, and the big money 1979 and 1980 Las Vegas titles at Caesar's Palace (destroying Connors in the 1979 final). Still he couldn't quite pull it off at the US Open. Yes he got unlucky there on a few occasions but close doesn't get you the cigar.
I do agree with abmk that Borg was unlucky that there were no big slow hard court events on the tour during his career, and that he would have been well suited to that surface, probably more than Sampras. Sill Sampras won over 80% of his career matches on hard courts during his career (fast or slow). He won 3 US Open titles during his prime, one before his prime and one after it when everyone was writing him off. On a fast hard court I would also take a peak Sampras over a peak Borg. We never saw Borg on a slow hard court of course so no comparison can be made there. In a way that's a good thing as the rapid increase in slow hard court events (the most damaging surface on the body) over the years has been one of the worst things to happen to tennis, in my opinion.
Obviously the gulf between Borg and Sampras on clay was as wide as an ocean, and far greater than the gap between them on any other surface. And overall Borg was far stronger on his weakest surface than Sampras. I do also think that Borg had tougher competition during his prime than Sampras did as well. However I don't think it is unreasonable to rank Sampras ahead of him on all fast surfaces, as facing superior competition cannot fully compensate for less big titles on a particular surface.
As I said before overall I still give the edge to Borg anyway because I considered his domination and versatility to have been more impressive than Sampras's.