I don't think you can go wrong with either, I would like to hear what Retrodog would say, maybe he will chime in and give some input. But for my money, I would probably get the Kohler. I have had two Kohlers, one in my Hustler and another in a lawn tractor that I sold about 6 years ago?.I never had one problem in the three years with my Kohler in the Hustler and I want to say my old lawn tractor is about 6 or 7 years old now and still going with the Kohler 20. I sold it to a neighbor down the street and he is still cutting away with it. I think the only problem that tractor has had is a fuel pump, other than that, still going.
There is also a mower mechanic that post in some of these forums and he recommends the Kohler engines over most engine choices, even the Kawasaki's. He seems to think that the Kohler is a better engine?
Hustler SuperDuty 42" 19hp Kawasaki KAI
Stihl BR-600 Magnum
AMSOIL In Everything I own