I have a gas powered G725 with the 3 cyl. Kubota engine and 61" deck and I have a diesel powered 22 hp 3 cyl. Kubota engine. After using them both, the only difference I can tell is that the gas powered unit runs smoother than most cars, and it also drinks gas just a little over twice as fast as the diesel model. I think I'm getting close to 1000 hours on the diesel unit and all I ever do is to change blades once a year (sharpen then during the year), and change the neoprene bushings that hold the deck to the tractor (about $4).
They will climb like a billy goat and ride so much better than a mid mount unit. I bought the diesel unit to mow 2 commercial jobs that would pay for the unit in 2 years for me. I liked the unit so well that I bought a 2nd one to go with it, but not before I test drove about every other quality unit available. Good luck!
