Diesels typically have slightly higher maintenance costs - larger oil capacity (though the oil usually isn't more expensive per qt) and much more frequent fuel filter changes. If you're going to use it in the winter (for plowing or snow blowing), you'll have to keep on top of potential fuel gelling issues. That usually means adding an anti-gelling additive, which is another fuel cost adder. Or figure out a way to keep the tractor warm (above 20deg F) when not in use. Diesels recycle fuel from the engine back to the tank, so once the engine starts heating up it's usually not a problem.
But the performance for tractor applications is so much better (torque!!!) plus they sound like tractors.
