Duo-therm is the name of a product made by Dometic. So, you're considering the replacement of a Dometic A/C with another Dometic A/C.
You would want to be sure that the capacity of the new unit is the same or larger. I would guess the one you have now is 12,500 BTU/h.
The "Brisk air" is the larger (standard profile) product. Airstream used low-profile air conditioners once these were available, and you probably have one. The three benefits of a low-profile air conditioner are: fuel economy while towing, bridge clearance, and exterior appearance -- the low-profile units blend better visually.
In general the A/C units equipped with electric heat do not provide enough heat to be of much use. If you really want electric heat in the A/C unit you may find that it's cheaper and far less work to add the heat strips to the unit you already have.
Typically the gas furnace produces around 8x as much heat as the heat strips. About the best you can hope for from the heat strips is to keep the trailer at 70 degrees when it's 60 degrees outside.
$500 for two A/C units is an OK deal if they're new 12,500 BTU/h units. If they're take-offs or the smaller 9,000 BTU/h units then it's probably not an especially good deal.