It is really the old "Ford or Chevy" debate. Since I sell Gravely, and Kawasaki and Kohler, I will give you my opinion. Go with the 27 Kohler. We have had no problems with the 27Hp Kohler. The big Kawasaki's seem to have a number of little nagging problems, and we see a lot of warranty work on the bigger Kawasaki engines. Seems to me, that once you go over about 13 or 14 hp on a Kawasaki, the dependability goes down.
In the 252Z application, I don't think it really matters much. If it were the 152Z application, I'd be inclined to go with the Kawasaki just because it fits the engine bay better. Same deal with Hustler.
But on stuff like Bob-Cat, Great Dane, ect, I like the Kohler a little better because it's much quieter than the Kawasaki with those exhaust systems.
But with the Gravely dealer here telling you he's seeing better reliability with the Kohler in that application might tip the scales....
I'm running with the 27 hp Kohler on my 260Z and have been very happy. It gives plenty of power and performance. The kaws start better in colder weather, but that should not be an issue of performance.
i would go with the 25hp kaw. it will be plenty of power. in my area, the kaw. is the talked about motor, and the problems are with the kohler, but like it was said, it's dodge/ford/chevy thing. in my opinion, it boils down to do you want 25 or 27 hp. i think 27 is a bit much for a 52" deck.