Hard or soft water for hot tub?

Hi everyone, I'm hoping someone can give me some insight as to using soft water or normal city (hard) water to fill the hot tub. I use potassium for the softener, not salt. Is it better to use the soft water to fill it? Will it affect the chemistry if I plan to use the smarterspa or saltron mini salt water chlorine generator?

Basically I just want to know which is best for a fill, softened water or hard water (it's city water so metals/iron are not an issue).

Thank you so much.