It boils down to preference and your needs... I've been in the bus conversion community for 20 yrs and have decided trucks meet my needs better.... here are some thoughts in random order as they occur to me:
Interior space:
40' bus gives you aprox 40' interior (less drivers seat area)
40' truck gives you aprox 30' interior space, you loose 10' in cab and hood. If you do go MDT you might cut that to 7'.
(bushpilot is correct HDT is superior to an MDT for a lot of reasons)
You do gain the possibility of a bunk above the cab which gives you an extra bed, however you don't actually recover all of your space because bunks don't go to the front of the engine... maybe you get 6' back at the most.
Safety:
Buses have a very good safety record, however they are less safe than a truck for two reasons I can think of.
Front engine puts a large engine between you and whatever you hit in an accidnet.
(there was a recent news story about a bus that got bisected by a freeway sign)
Front engine puts more weight up front which (my opinion only.... improves handling characteristics by giving you more understeer rather than oversteer in loss of traction situations)
Maneuverability:
Buses tend to turn sharper than a truck, (MDT's and newer HDT's tend to turn sharper than older ones)
Many trucks have a shorter wheelbase (with more rear overhang) which improves break over angle and turning radius... you do need to be vigilant of the rear swing. Some trucks conversions have wheelbases similar to a bus.
Convertibility:
With the bus you have the drive train in the rear so you can get a basement that goes all the way across, or you can lower your living quarters into the basement for creative designs/ hot tub, whatever.
It is often necessary to raise the roof on the bus (depending on year/model)
The buses I've worked on have had less right angles and a lot more custom fabrication was required.
With a truck box everything is square and you get more usable interior space.
Capacity and reliability:
The buses I've had experience with always seemed to be at their limit with respect to weight and handling.
The trucks are typically designed to handle a lot more weight, and although you might have to watch your front axle weights the truck drive train is a lot more robust.
Coaches on bus chassis *might* have an overheating and fire problem, (I'm not gonna say how I became aware of this, however do some research on OEM manufacturers using bus chassis, lawsuits and settlements, and you'll find some interesting reading about heat, fires and rear engines.... google the names of the manufacturer of the engine, chassis, coach and "fire" to find the info)
The truck engines seem to stay cool with the engine up front and the radiator facing forward.
The trucks can tow a lot more weight.
I'm sure there are other considerations... perhaps ease of obtaining service, campground acceptability and perception, etc.
