From a pure grunt perspective, see if you can find the maximum torque output rating for the Briggs engines. You'll find everything you want to know about the Kohler engines on their website, even the technical manuals are available free.
When I was looking for a small LT - the single cylinder Kohler, although only rated 16hp - had just as much peak torque as the twin cylinder 17hp Kawasaki that would have been the next engine up. So far that little Kohler has run very well.