Think of it this way-
With Sundance you get top of the line quality, with all the bells and whistles. With Sweetwater you also get good quailty, just not all the latest and greatest. Sweetwater will be a good, solid, reliable spa, for less money than Sundance.
There are differences in the jets, and pumps, and electronics. Also, Sundance has more options for skirting- cedar, redwood, or resin. And Sundance has more color options for the tub itself.
You won't go wrong with either Brand.