I like similarities. First, I'd like the two competitors to be of the same sex (female). I know there's a market for mixed wrestling, but I'm not part of it. Crossing ethnic (aka racial) lines does nothing for me either.
Second, it's the competition that I enjoy. In fact, I really don't care about a clear-cut winner -- for me the turn-on is watching two highly competitive women going toe to toe. I believe that you're more likely to get great competition when the women are evenly matched in skill level, height and weight, even age. So I'm looking for near clones and the thicker the better.