Welcome to our forum.
Here is my take,.. if it is full foamed, it will cost less to operate, espesially in your climate. Do you know that approximately 15% of the heat loss is from the sides and bottom of the spa? Should you require service due to a leak in the full foam, it most likely will be covered under the manufacturers warratee.The odds are in the favor of the full foamed spa manufacturer, they ARE more efficient, and they will cost you less for the duration of ownership. If you only save $4.00 per month, due to the foam insulation times 15 years of ownership, you will save $720.00 in just that.Good luck...
I am surprised that the Sundance is the lowest based on reputations and more consistent recommendations...Wet test and see what feels the best...If in the end they all work well for you, than I would go with the Marquis.....uhhhhh I mean Sundance...
We looked at a number of spas and decided on the Caldera tub. I found this web site to be extremely helpful in my decision and my advice is to find the tub that fits you and historically been trouble free. Then buy the one you like the best. I consider myself to be somewhat frugle however an additional thousand dollars or so is worth all the time you'll spend in your tub. I have used my tub in sub zero weather and when you're sitting in your tub in total relaxation you'll be glad you purchased the tub you really wanted. It's a great feeling.......Good Luck.TC