Many think diesel is the way to go. But, you can get 2000 hours out of one those kawasaki gassers in the JD x700s. For my use level, that's like 35 years. You will give up some torque. But, you can operate it quicker with much shorter warmups, and it will be considerably quieter too. It's also like $1500 cheaper to buy. And a little cheaper to maintain because the gasser isn't as fussy about fuel filters and fuel additives.
Diesels use less fuel, but the fuel cost more. So when you take into account the fuel cost difference combined with the higher intial cost, the gasser is a bit more economical, at least for the first 20 years or so.
I'm at 1050 hours, year 19 on my gasser. She runs great. (JD425).
The diesel versions of the X700 make the most sense when you are pushing their hp limits, like with the 60" mower deck, or a big tiller, or the 54" snowblower. Power to ground, I doubt you'd see a difference because you'd most likely lose traction before reaching hp/torque limits.
However, if I were to go gas in one of these expensive machines, I'd probably only consider the fuel injected ones because of the ethanol they now put in gasoline. Ethanol just kills carburetors unless you consistently treat your fuel.