Well they aren't any 'absolute' answers to your questions...the honest answer is "it depends".
Really, eveyone sort of finds what suits them whether from a financial standpoint, space standpoint, use standpoint, aesthetic standpoint, etc.
I personally chose a school bus with a conventional nose because I want to be able to travel dirt and gravel roads and wanted the underbody clearance and I don't want to have to crawl into the front, into the rear, or under the bus to get to the engine to repair or maintain it. I like to flip the hood and have the whole shebang right there ( I ain't gettin' any younger!).
I specifically looked for a bus with an International chassis so I could get the DT466 engine; I've never run across anyone that didn't think this wasn't a great engine. Because it has so much torque it typically gets mated to a heavier duty transmission (an MT645 auto in my case) than most schoolies.
I thought hard about an over-the-road coach like and MCI or Eagle or such; great ride, lots of storage underneath and good looks. But I didn't want to leave the pavement on some back road and start worrying about getting stuck where a very large (and expensive) wrecker would have to pull me out. I also didn't want to worry about the overhead clearance quite so much or the large turning radius.
I like the looks of the RTS buses but they don't have any more road clearance than the OTR coaches and they don't have underfloor storage; for me it was the worst of both worlds.
With the school bus I get an inexpensive, robust and easy to work on vehicle that's been designed with ease of maintenance and simplicity in mind. It has a short (by comparison) turning radius, it's designed to operate anywhere school kids live and it's easy to get parts for.
But, I wasn't looking for a road burner; I intended to find nice out of the way locations to hang out for a while and relax. If I intended to travel
a lot more and chew up some miles on an ongoing basis I would have looked at an OTR bus more.
Diesel versus gas is an argument with no answer; for every one of us the answer has to come from our intended use and personal viewpoint. If you're really going to put 10,000 miles a year or more on the bus, diesel probably makes more sense than for a bus that will get 2,000 miles a year. Diesels don't take as kindly to sitting around as gas engines do and the maintenance items for them are (and on them is) more expensive (you're changing gallons of oil; not quarts!). It takes a fair amount of driving to pay for the extra costs involved with a diesel engine; in other words, you can buy a lot of gasoline for the money difference if you don't drive that much.
Gas engines are quieter, smoother, have more power and are (generally) easier to work on. I got a diesel since I'm in it for the long term and wanted an engine that would get better fuel economy and last a very long time (the DT466 is reputed to be a 500,000 mile engine; although I'll never find out if that's true!). I'm not sure a gas bus wouldn't have been a better choice for me but part of the decision is just satifying the 'want' inside. I really 'wanted' a diesel so I got one; the first oil change and lube was $175.
My bus gets just over 9 mpg (we'll see if that holds true after the conversion's finished!); I believe that the typical gas bus probably gets in the 5 mpg range. Sounds like a lot of difference until you add the extra initial money up front and the extra costs for maintenance. You'll just have to do the math for your situation.
Oh yeah, the sound deadning material to make the bus more livable at road speed is several hundred dollars! I think I could have avoided that altogether with a gas bus.
You pays your money and you takes your choice...have fun!