Both Unicel and Pleatco are perfectly fine filters. Yes, most tub makers use one of these two brands for their spas. Unicel is the larger of the two, but I have never seen an issue with Pleatco. I used to replace swimming pool filters with Unicel all the time. Of course, those filters come with Unicel right off the bat.
Don't know about the special Sundance filter though. Any Sundance dealers could probably chime in on that one. In addition, the Hot Spring Tri-X is proprietary. You can only get that from a Hot Spring dealer.
Other than that, Unicel makes a replacement for nearly every filter out there.