I have the kohler 15 on my exmark metro hp. I bought the metro vs. the turf tracer because I mainly use my Lazer. If you plan on getting a ztr in the future you may want to consider this. I had Kaw engines before and switched to Kohler you can probably use the search and find more topics on engines.
I bought a Turf Tracer HP at a show in February 48"/17Kaw for $4299. I would recommend the 17HP over the 15HP. Get into some taller grass (especially when mulching) and you will be glad you did. My general rule of thumb is to get the most power available as long as the price is reasonable. Also, definately go for the Kawasaki.
As far as the inner workings of a Kohler vs. a Kawi you'll have to consult one of the resident gearheads, but from a practical viewpoint (ie. how does it cut?) our Ransomes 54" gear-driven WB has a 15hp Kohler we've got no complaints thus far. Even with double-blades and all of the tall wet grass this year.
I have two turf tracers. One with a 15 and one with a 17. They are both 48". I regret buying the smaller engine. It runs good, but with the hydro and a velkie, it struggles a bit. The kohler engine has been reliable, but I recommend the 17 Kaw. Exmark is coming out with a new walkbehind in January, it looks pretty cool, you may want to wait.