Both Swirl Away and Spa Purge are good products. We used to sell only Swirl Away, but switched to Purge for the main reason that Swirl Away wasn't compatible with biguanide systems like Soft Soak and BaquaSpa.
Purge does need to have the water temp be 95 or higher, where I don't remember Swirl having a requirement like that. However, after using both products myself, I think Purge does a better job.
Regardless, for a bromine or chlorine system, they both will get the job done. It's really just personal preference.