ON paper, they are so so similar. I remember when Watkins came out with Marino (caldera) and the Relay (Hot Spring) back in 2010. When they came out with it, they were combating the recession and a depressed market.
About a year or two later, Jacuzzi/Sundance literally copied the models with slight changes to not infringe on patents. (I was a Hot Spring/Jacuzzi dealer back then.)
I know that the cost back then was also higher on the Jacuzzi because of ClearRay being standard on everything. I personally prefer ozone to UV-C by quite a bit. However, both do not come with ozone standard so that is kind of a non-factor in this decision.
Due to my experience with the companies on the backend side of the business, I am more partial to Caldera