Huge Diesel Fan here. Even if it were just because they sound so much cooler when you first start them up.
The biggest plus for the diesel's isn't so much that it makes more torque, but that their torque curve is much flatter. If you always run your gas engine at full throttle and never lose any RPM's, then the diesel won't benefit you much from a power point of view, but it will save you a LOT of gas. Partly because they burn less at WOT, but also because you won't have to run WOT all the time. You'll have plenty of power at 70%.
If you're running more than 30 gallons of gas through your mower a week, and a diesel will take 15 gallons, how long does it take for a more durable engine that's saving you $250 a month in gas to pay for itself? Probably not long in Florida, but might be a while in Vermont with the shorter mowing season.