That's really tough. At first, I wanted to say John Wayne because he was a big, tough, burly guy, so he might be hard to take down.
But, I think that my final answer is: Clint Eastwood would be the last man standing. He's a wiley sonnabitch and a darn fine director. I think that Eastwood (the sexier of the two) would win.
Here's how it would happen, Eastwood...being a gentleman...would walk up behind an ususpecting Wayne and say, "I'll give you one chance to give yourself up to the law..." and Wayne would spin around reaching for his pistol, but Eastwood would shoot Wayne in the chest with a rifle.
However, if fighting in a close combat situation, with knives or fists, it could go either way. Wayne was stronger, but Eastwood was quick and quick-witted.
My money's on Eastwood.