The same rules for heating and air conditioning that apply to homes would also apply to RV's.
Basically, the most efficient unit would run 24/7 on the most extreme day of the year, yet keep the temperature where you wish it.
If the unit used a smaller fan, there would be less noise.
The cycling is where inefficiencies come in, and this was very true in this neck of the woods with heating 20-40 years ago, where they just put in BIG units and new they would do the trick.
While a larger A/C unit would cool things quicker, it would cycle in and out more often. A smaller unit may not be as quick to drop the temps, but it would not take that long, and would easily maintain the temp. With the good insulation, I would not hesitate to use the 9,000 BTU. Actually, one that has received quite good reviews is the Coleman Polar Cub 9201.