My interest in, and knowledge of tennis is negligible, but a cursory examination would suggest these two are closely matched.

Unless of course, the Borg referred to is that Star Trek nonsense, in which case Federer kicks its ass.

On the clay, Borg would dominate Mr. Fed. Roger wouldn't have too much trouble on hardcourt with Borg. On grass surface, my guess is that Borg would've beaten him at least 60% of the time so, biased reviewer that I am, I'd give a slight edge to Borg.
Someone dust off the time machine.

Good match. Control, cool-headedness, technical wizardry of Borg dominated the era that prededed the scrappy, gritty and pugnacious McEnroe. I think Mac carried the day in the matches between the two (not sure of that though) but they met when one was on the ascent and the other was in his later days and showing signs of having lost his edge.