I don't use one because I I've in Florida and the coldest my apartment ever gets is about 70 degrees. The ambient temperatures in my cages never gets below 75 degrees.
Using both give you the most control in your cage.
You can use aspen or reptibark, but the layer will need to be very thin.. less than 1/2 thick. I am currently using a RHP in my Dumeril's Boa's cage (dumeril's boas burrow in their substrate so it has to be a thick layer)
The main reason to get a RHP is top keep the ambient air temperature in the cage above 75 degrees. If your room temperature stays high enough for that year round (like mine does) then a RHP is not necessary.