There is no comparison...
Sundance makes very high quality spas, has reputable dealers, and are built to last.
Superior Spas are disposable generic spas that I've seen being sold at the state fair and on the side of the road by roaming "factory reps". Some of the worst quality spas I've ever seen in the tradition of Sunbelt, Morgan, ALPS, and Aries. I've never seen a spa made by one of those companies that I'd take if someone gave it to me.
If those are your only two choices, Sundance by a landslide.
Good luck!
Terminator