Welcome Sandra to our National RV site. I would think if you were to do much hot weather camping the 15,000 btu unit will be much appreciated. It pays to have as many bells and whistles as you can afford at the time of purchase. If you try to add them later it will cost you much, much, more.
I don't think you will go wrong with your Dolphine, the prices are very attractive now. Make the dealer throw in a 5 yr warranty with the coach for some security.