Sampras was better than Borg on 3 out of the 4 surfaces; grass, hard and carpet/indoors which is a big plus in his favour.
Plus his longevity was outstanding. I've always believed that his feat of winning slam titles as a teenager, in his 20s and in his 30s (Rosewall is the only other man to have done that) is one of the most underrated achievements in tennis history. In fact I believe that Sampras's longevity is actually better than Agassi's, which was hugely aided by him half-assing it and not fully dedicating himself to the sport throughout most of his 20s.
As far as domination goes, Sampras was the year end no. 1 from 1993-1998, and Borg was the best for 3 years from 1978-1980, so you can argue that Sampras was far more dominant. However if you look at the quality of Borg's prime years, they were better than Sampras's in my opinion. I think that Borg's 1979 and 1980 were both better seasons than any years that Sampras ever had. His 1978 also rivals any of Sampras's best seasons. Borg's 1976 and 1977 seasons when he wasn't the best player in the world, were still better than Sampras's 1996 and 1998 seasons when he finished as the year end no. 1. Sampras was never really as dominant over a stretch of years as Borg was from 1978-1980.
I do think it is very close between them and many writers and tennis historians considered Borg to be greater than Sampras and vice versa. Neither of those two players are on the same tier as Federer or Laver in my opinion though.