I’ve got the 28hp Kubota diesel in mine. I’d sure hate to feed the 30 hp gasser.
It's not as bad as you'd think. The gas units are lighter, and more responsive under changing loads. Back when all the commercial zero turns were heavy, there was a clear benefit to diesel... but gas still outrun them in productivity.
Nowadays... unless you already stock a load of diesel for your equipment (tractors, etc.), or live in a place where diesel isn't so much more expensive than gas... there's really no reason to buy a diesel, aside from longevity.
Where I live (southwest Virginia), diesel runs about 30-35% more than gas. 93 octane "high test" runs about $3gal, compared to #2 diesel at $4.10 gal. I had a Grasshopper 928D, with the 28hp 1105 Kubota 3cyl. I now have an older eXmark Lazer Z with a 27hp liquid cooled Kawasaki... the heaviest gas model they made. There's NO COMPARISON is bottom line... the gas model is better.
The Grasshopper was a slow dog uphill, and not nearly as responsive in the turns. Even though it burned less gallons, the price to mow 7.5 acres (5.5 acre lawn with two auxiliary lots) is about the same with gas being much cheaper. If I would run cheap 87-octane, the gas unit would be cheaper, AND faster.
Aside from being able to put 5-6,000hrs on a diesel without a rebuild... there's really no point. ESPECIALLY when you consider that a gas unit with equivalent hydros will see hydro life last nearly twice as long. All the added weight of a diesel really stresses the hydros on hills and direction changes.