When it comes to comfort systems like AC/Heat more is always better. A lot of builders go cheap on these systems (which is a good indicator of the overall quality or lack thereof). Lots of 30'+ RV's out there with just 13,500 BTU AC's and less than 20,000 BTU furnaces. Kind of makes you wonder what else they skimped on. :R
Our 24' Class C has a 35,000 BTU furnace and a separate system of heated tanks. While we do camp throughout the winters our winters here in the west are mild and yet that 35K furnace still gets a workout every trip. Overall a larger system will be more efficient. I recommend the OP goes with the largest system available.
:C