All RV air conditioners are noisy. The 13,500 BTU unit will run longer - which may actually be less irritating when you're asleep than having the bigger unit cycling on and off, waking you every time it starts up. A unit that is not constantly switching on and off may also last longer. OTOH a 15,000 unit on a 100 degree day will cool faster and perhaps to a lower temp than a 13,500 can achieve. Where you camp is important too. In Southern TX - go for the bigger one, in the hills or woods where temperatures are more moderate, the smaller one makes sense.
Personally - talk to someone like Lewster and find out which units have the best durability and lowest noise level, and best ability to direct air into the both ends of the camper - then choose that brand and size recommended.
Today is a gift, that's why they call it the present.