I do believe that the 05 Sundance is going to a 35 gallon per minute circ. pump. That will increase the flow rate considerably. The Marquis spa does not use a circ. pump, but it runs a few hours each day and 30 minutes after each soak. The filtration system is more than adequate for any size spa. The Sundance also uses the Micro-Clean Filtration which is supposed to clean to a smaller micron.
You'll be OK with either one...
When I owned a Marquis Escape, which is comparable to the Euphoria in size,it was costing me about 28 dollars a month in the winter @ 12 cents per kilowatt hour.