There are a number of arguments on either side of this fence.
Gas:
- cheaper parts, oil changes, etc.
- less noise
- any repair shop should be able to work on it
Diesel:
- more efficient (MPG)
- more robust (lasts longer, less powertrain work required, in theory)
- more torque / pulling power
I'm sure there are others. Anyway, for our weekend warrior short bus, I chose gas, and it's fine so far (still mid-conversion), but it was also relatively low-mileage and in great mechanical shape when I got it.
Most people tend to lean towards the diesels, especially in the big buses.
Chris