a/c vs. heat pump

[bottomacher]

Does anyone know of any definite advantage or disadvantage of either in a rooftop model? Thanks

[TomC]

Straight A/C is simple and for heat they add an electric heating strip that works, if you give it time to warm up and close the vents to slow down the flow of air.  The electric heating strip is rated at about 5,300btu/hr and pulls around 1,500 watts.
The heat pump has a reverser valve for the freon in it that throws the cooled air outside and the heated air inside.  The advantage-for that same approximate 1,500 watts that was used for the electric heat, the heat pump will give around 12,000btu/hr of heat-but only down to about an outside temperature of 40 degrees (not much heat to capture below that to pump into the bus).  So many heat pumps will also have the heating strip for when it is too cool outside for the heat pump to work. 
Personally-I like simplicity.  I have three 13,500btu/hr Coleman roof tops with heating strips.  While they take a while to start pumping out the heat, they do work and can make a sauna inside.  In the 16 years I've had the 3 roof tops, the only thing I've had to do (up and beyond cleaning the filters) is to replace the front outside shroud.  Good Luck, TomC