I say the astronauts would win, not the cavemen. Why? Well, think about it. Once you piss 'em off enough, they'll still be able to be just as ferocious as the cavemen, so that should put them on at least equal footing. Plus, the astronauts will be wearing space suits that they can use to frighten and intimidate the cavemen with. Then once the cavemen are off-guard, the astros can surprise 'em. 🙂
But really, the whole debate isn't quite so silly as it at first sounds. As one person put it, it's sort of magic vs. tech. Or it's primal instinct vs. modern day, cultured people.
-->Warning--you are now entering psychobabble opinion zone<--
Choosing the cavemen to win means you feel that, ultimately, the human nature is to kill, and just to kill and that's all. Choosing the astronauts to win means you believe that humans can somehow overcome their powerful id and that we're more than just base animals. This explains Angel's and Spike's choosings.
In `Forbidden Planet', I think the astronauts sort of win in the end, but I forget. Of course, they're fighting powerful id monsters, not cavemen so it's not quite the same.