Goku vs Superman

[Animeniax]

By the same token, it would be out of Superman's character to treat him like a villain.

Maybe if (pre-non-American citizen) Superman was commanded by the US President to deal with what the leadership considered a potential alien threat in Goku, then Superman would have to fight him. Or we could remove all those variables and just say this is a tournament style fight.

I wonder why they have to factor in the personality of those involved. Can't it be a mere test of potential without all that drama?

Their personality plays a big part in their fighting ability. You can have all the power in the universe, but without the right mindset to use it (and all the ways to use it to your advantage), it's meaningless. I watch a lot of pro fighting, and the lack of killer instinct and heart is what separates the pretenders from the champions.

spirit bomb goes out the window, as it's unable to hurt superman (being pure of heart and everything else). I don't consider the possibility of taking inside the energy of the spirit bomb, I don't consider it canon to dragon ball.

That's one of those details I forgot about. The spirit bomb only works against evil (I'm reading up on this to be sure).

edit: the Dragonball wiki is slightly contradictory about the spirit bomb:

one must have a pure heart so they can manipulate and gather energy, otherwise the move can backfire and possibly hurt or kill the user. Likewise, once a Spirit Bomb has been fired, it is possible for one with a pure heart to deflect the technique, as Goku clarifies to Gohan and Krillin. However, it should be noted that although a person can be pure good or pure evil of heart, a Spirit Bomb created by a pure good person can not be deflected by a pure evil person, as the two opposite energies cancel each other out and one engulfs the other. This is shown when Frieza (a being of pure evil) is not able to deflect the Spirit Bomb, although being able to survive it through brute endurance. However, the Spirit Bomb is capable of harming anyone, regardless of morality.