Taking a moment to raise my powerlevel:
Both fighters hold back. Significantly. If Goku ever really fought without restraint, he'd explode the planet he was on, if not mar it significantly (Cell Games, Namek). Calling out even more power (SSJ3) is a drain as such that even a god's magic can't sustain his existence proper (24 hours granted life ticket shorten to forty minutes or so--and people all over the COSMOS--both sides--felt the surge he was putting out). Not to mention most of his techniques at even SSJ1 would make short work of the numerous hax gadgets DC and Marvel cook up and dare to call a challenge any day.
On the flip side is the Man of Steel...who holds back. Significantly. Like Spider-Man--in truth, were both heroes to go all out, opponents would die. End of story. Superman's strength under a yellow star is damn-near uncontested by almost every character EVER (save a small handful, and I'm thinking all publications, not just DC). He could punch a hole into anything you could think of, and is probably the only being without time abilities that could hang possibly toe-to-toe with the Flash. I can think of a handful of Marvel entities (who are consistently more fuckpowerful) that Superman would have to work at, unrestrained, to beat. And by that I mean four. The guy is invincible and it would take something reality-bending to hurt him. Doomsday somehow qualified. DC agreed, bitch at them about it.
If both Goku and Supes are coming out the gate with all guns blazing, it's likely that Superman will have the victory. Unless he, for some oddball reason, DOESN'T punch Goku directly in the head or heart. At which point he might just not make it out alive. The point is no matter the circumstance, Goku isn't walking away clean, and Kal El is gonna feel it for days.