As a lawn care professional that has 12 years experience, I can tell you that they are both good engines. John Deere doesn't cut costs, the engines are nearly identical to its normal counterpart, usually different cranks, valves, etc. Deere is the only manufacture I know of that custom orders their engines, all other manufactures use the normal Kawasaki engine. That being said, it was common in Deere's because they still used horizontal shaft engines as opposed to vertical shaft.
Kohler engines are built stronger than most others. They "overbuild" them, if you take one apart and compare it to a Kawasaki you'll see. Kohler's typically make it longer on life as well compared to Kawasaki. I personally haven't had either over a 1000 hours, so from personal experience I can't say that I've used a Kohler or Kawasaki longer. Typically when a Kawasaki has an internal failure, you need a short block. If a Kohler has the same problem, you typically only replace the bad part and off you go.
I'm not saying either one is better, but for longevity I would say Kohler will last longer and stand behind their product better. I've worked on both and used both, think they are great.
Gravely is one of those brands that I tried at one time and was not impressed. They have a short expected service life of less than 2000 hours. They used cast iron spindles, sealed bearings, and had less extra cooling on engines and hydro pumps/motors. Nowadays Ariens made a big turn around on Gravely, especially after the acquisition of ACI. Gravely is a lot better than they used to be, that's for sure. Most of the brands are really equal, some just have some features others don't.