Cable actually let go of the island, btw... Rachel took over for him midway through the fight, becasue he couldn't keep all of it up. Both were repairing damage. Surfer blasted him a total of once in the whole fight, even though that's pretty much how he fights everyone who he's really trying to put down. It's pretty clear that both were holding back.
Why was Cable though? He was extremely dedicated, and I doubt he'd hold back from hurting someone else who was trying to stop him. It's even possible that he wasn't trying to fight SS because he knew he couldn't win. The whole time, he was asking Surfer why SS wouldn't let him finish his goal and whatnot... It's always a possiblity.
All in all, Surfer wins, with or without his godly speed. His matter manipulation is well beyond Cable's, and he has been shown to have some good psionic defense before. (again, he didn't even know the aliens were there... he was caught unawares, and so wasn't trying to resist when they balsted him) Cable is much less durable. Surfer has a variety of basically instant kill moves that he could pull, while a win for Cable would require a good bit of work.