I would go with with either. I think 13,500 would be fine. That is what I have in a 28' camper. We camp in the summer, usually doesn't get over 105 degrees in our area. In full sun and 100 degrees, the a/c is running most of the time to keep about 70 degrees inside.
I would think with your smaller camper, and potentially hotter outside temps, I would think the smaller a/c would be comfortable. I wouldn't miss out on a good deal of the a/c difference.
On the other hand, your spending thousands on a camper. For an extra $150 if you can eliminate the potential to be uncomfortable, I would get the bigger a/c.
Personal preference, I think. Good luck
2016 Road Warrior 355
2015 Ram 3500 Cummins SRW 4x4 MegaCab ShortBed
2011 F150 ecoboost - Traded
2015 Jayflight 28 BHBE (sold)
2009 Jayflight 26 BH (sold)