For all practical purposes a diesel engine is the best choice for an engine in a full size bus.
I have driven buses with big gas engines. Not only do they not have as much go but you can really use up a lot of gas trying to move a full size bus.
Back in the day the MV446 gas V-8's in International chassis buses got 3-4 MPG. The older IHC LV478 powered buses had stick shifts so they got a little better but I wouldn't call 4-5 MPG good. And if you really wanted to use up gas quickly the Ford 534 V-8 got about 1 MPG when you really got it working hard.
We also had seven Gillig Schoolcoaches that were identical in every respect except six had the Cummins C-190 190 HP diesel and one had the IHC RD501 185 HP gas engine. On every hill the gas bus was one gear lower and it got half the fuel mileage the diesels got.
In addition, those gas powered buses were doing all they could to get the buses to go 60 MPH on the flat. As soon as you hit a hill you would be dropping gears. 45-50 MPH on the hills on I-5 in WA state was about all you could expect. And those hills are not especially steep.
In small buses the differences are not that much different when comparing gas to diesel. E-350/450 buses with the V-10 get about 12 MPG. The same bus with the 6.0L diesel get about 14 MPG. The G-3500/4500 with the 6.0L get about 13 MPG and with the Duramax diesel get about 15 MPG.
I have no real experience with the full size GM buses with the newer fuel injected Tonawanda 366 gas V-8's. We had a few 366 buses with carbs back in the day that were pretty good buses but their mileage was about the same as the IHC buses. The newer ones with the electronics most probably had fewer drivability issues but the fuel mileage still would not be much better than 5-6 MPG.
Most Cummins 5.9L/ISB powered buses get 10+ MPG.
Yes diesel engines cost a lot more to fix. Yes diesels cost more to service. But when they get almost 2x the fuel mileage it is hard to say no to a diesel.