I'm sure you've decided by now but thought I would share some thoughts nonetheless as I have insight into both of these brands.
Sundance Spas are a solid made hot tub but their weakness is that you generally don't nearly as much bang for your buck as most other brands to include Four Winds Spas. On Sundance the pumps are smaller and thus the jets tend to also be smaller. The Clear Ray UV system is a clever idea but mostly to the benefit of Sundance and the dealers since you have to buy the bulb every year at about $100 a pop. Also, the technology doesn't 'kill' anything but rather neutralizes it so that it cannot reproduce. It is a great marketing story, which Sundance does well, but really it is not any better than any other option.
Four Winds has gone through some massive changes over the last couple of years with some new investors getting into the mix. The spas have been vastly updated and honestly look amazing compared to anything in the past. The Frog System is not a dated feature but a feature that works and is very easy to use and even with Sundance the Clear Ray still requires that you still add chemicals into the water regularly. A better comparison to the Clear Ray would be a CD Ozonator which doesn't require you to replace it every year (about five years) and kills anything you want dead on contact and doesn't just neutralize it.
Quality is no doubt in the eye of the beholder since the ironic thing is that nearly every hot tub made is made of parts from a few source manufacturers. So, as an example, the surface of the spa (acrylic) has two sources in the USA and everyone buys from them. So to say that one hot tub manufacturer has a great surface and another doesn't really makes no sense.
Marketing it important but don't be tricked by smoke in mirrors.