As someone who's been with a few species all I can say is that it really depends on the individual. For the most part any female with compatible parts will provide the necessary friction. The bigger difference that makes the most importance is how one feels about a particular species. I'm not into equines, I've tried, they're just not for me but that doesn't mean anything outside of my own head.
If one is attracted to bitches, bitches will be best....
if mares, mares will be best...
if cows, cows will be best...
if women, women will be best...