I loved watching Borg on clay and I love watching Nadal on clay these days. It's pretty clear that they rank at the top among the Game's clay courters, with Rosewall somewhat in the conversation as well, per many.
Here are some stats on Borg's clay career below. See the 30 clay titles he racked up between 1974-1981 below and his clay titles by year. His 30 clay titles were among 64 total titles. 23 of those 64 titles were indoors and he had 5 hard court titles. Among his 30 clay titles, he amassed 6 FO titles and also other prestigious clay titles including two Italian Opens and three Monte Carlo titles. 30 clay titles over 7 years means an average rate of 4.3 clay titles each year. Nadal now has 25 clay titles, out of 36 total, amassed over 6 years from 2004-2009. That's also a rate of a little over 4 clay titles per year for Nadal (vs. 4.3 for Borg for 7 years from 1974-1981). So, their "rates" of clay titles are similar. Nadal is now 181-16 so far on clay, while Borg was 245-39 during his career on clay.
Here are the number of clay titles Borg won by year:
1974-4
1975-3
1976-2
1977-5
1978-4
1979-5
1980-4
1981-3
Here are his clay court titles:
French Open 1974, 1975, 1978, 1979, 1980, 1981
Geneva, Switzerland 1981
Stuggart, Germany 1981
Monte Carlo 1977, 1979, 1980
Nice, France 1977, 1980
Pepsi Grand Slam 1977, 1978, 1979, 1980
Palermo, Italy 1979
Bastad, Sweden 1974, 1978, 1979
Boston, Massachusetts 1974, 1975, 1976
Italian Open 1974, 1978
Barcelona, Spain 1975, 1977
Madrid, Spain 1977
Dusseldorf, Germany 1976