The 6.6 is a bulletproof engine. IT's wimpy, but so are the other two you are comparing. It was a tractor engine before it was a highway service engine.
The 6.6 liter is commonly known as the new holland, or the brazialian ford.
parts are expensive, that's about the only downside.
I have this motor, with the at545 auto allison and get 8.5 mpg in a 72 passenger.
I also paid only $775.00 for my bus. It had 254K miles on it when i purchaeed it. Now has about 25K more.
Quis custodiet ipsos custodes (who will watch the watchmen?)