Tough call on prime Laver versus prime Borg.
Recalling that the two players who gave Borg the most trouble were Panatta and McEnroe, I am inclined to say that a great serve-and-volleyer, like Laver, would give Borg a lot of trouble, particularly on fast old Wimbledon grass. (This is in no way underestimating Borg's grass play.)
On clay, I am tempted to give the benefit of the doubt to Borg: that slow RG clay would allow him to run down so many shots. Laver would have to place everything quite perfectly--otherwise it would come back. Then Laver would get impatient and start pressing more aggressively. In that case Laver either gets better or he gets frustrated and goes for too much (a la Hilton Head).
x2 on the spot on analysis.
Laver, IMO, was more versatile than McEnroe and better from the baseline. With Panatta, he was just a bad match up for Borg, but IMO Panatta would have been O-fer against Laver...it's all about match ups.
On clay, I also agree. Borg, with wood and slower clay and pressureless balls of the era, was unbeatable and the best ever on clay. If they played 10 times on clay, Borg would probably win 8. On grass, old grass with wood, Laver wins 5. On hard courts, I think Laver wins 9. Indoors is a tossup.
I also agree heartily with the comment that while both men are class acts, both considered themselves hands down the best on tour when they were in their prime. I think the realization the McEnroe was making headway really woke Borg and he just had put so much so long into his tennis that he was ready for a break.
There is a story about a player who was playing Borg, I can't remember his name, but he was basically a journeyman. They were playing indoors I think. Anyway, the guy's family had come to see him play Borg. He wasn't doing too well being down 6-0, 5-0 with Borg about to serve for the match. On the changeover, he explained to Borg that his family was there and he knew Borg was going to win, but if Borg could just let him win a game or two, it'd be nice.
Rather than let the guy win a point, Borg double faulted 4 times to give him a game. He then proceeded to break the guy for a 6-0, 6-1 win.