I'm going to go against the grain here and say gas. My bus is gas but then again I don't drive it more than 1,000 miles a year so it should last 50 years at this rate
Seriously though, the one thing I didn't like with diesel is the cost of replacing any part in your engine, it is extremely expensive. Depends on what you're looking for. Heck if I ever do a bus again I may do deisel, it is nice for deisel owners to not worrying about pesky things like inspection.