Tennis was in its heyday back in Borg's time - Borg, McEnroe, Connors, Evert, Navratilova. Tennis was much more popular - participation wise. Borg was VERY big - given the limited TV and lack of internet/social media that there is today. And to the poster above, Borg was good-looking - still is (this from a female). But Federer is beloved worldwide - no comparison - much longer career, beautiful tennis and better business branding.
Borg was also much more isolated - kinda like Sampras while Fed is open, easy-going, friendly - enjoys the tennis life so he'll be able to have a very, very long career. I do think that the establishment was not kind to Borg when he tried to make his comeback - imagine Wimbledon wanting a 5 time Wimbledon champion to go through qualifying rounds to play at Wimbledon - instead of granting him a wildcard - that still pisses me off. Huge Borg fan here (and Fed fan).