Borg was probably more ahead of his generation than Courier, so that will make it closer. But Couriers forehand would be a hell of a lot bigger than any forehand in that era. So I would put my money on courier.
You think Courier with a wooden raquet would have a forehand which could hurt Borg?
Nadal with 90s raquets wouldnt lose a set against Courier, Courier with 70s raquet wouldnt win a set against Borg and Nadal with 70s raquet would play endless 8 hour epics against Borg.....