Superman is one of the strongest heroes and had the power to defeat Goku. Sure Goku could of let Superman die to the kryptonite, but he wanted to face Superman at his strongest.

I know nothing of Superman, but it's usually implied he's all but invincible in some of his incarnations, and is basically as strong as he needs to be.

Goku is just really, really, strong. He was at a planet destroying level by the end of the Saiyan Saga, probably, and going by fan power levels, he's probably at least 50,000 times stronger than that by the end of the Buu Saga at full power.. He definitely has a finite power level, though, and can easily be beaten by someone stronger.

So it seems like a character written around his victory vs. a more definite character, which doesn't really make for a good fight, honestly.