I've been to the watkins plant. They promote both their Hot Spring Line and Caldera Line as "premium" spas. They are constructed very similarly, when it comes to the frame and equipment compartment. They do have their differences though. All of the lines of spas that watkins makes are fine products and are well constructed. If you are looking for a premium spa I would also suggest that you take a look at Dimension One, Marquis, Sundance, to name a few others to compare with.
Jason,
Store Manager for a Dimension One & Caldera Dealer