Chlorine or bromine in a hot tub?

They are cousins in the same chemical family and do the same job by different routes. Spent chlorine turns into chloramines, which smell and sanitize poorly; spent bromine turns into bromamines, which have no sharp smell and keep working. That is why bromine tubs tend to smell less like a pool. Chlorine acts at once, so you dose it as needed; bromine tablets dissolve slowly and take a few days to build a steady reading.

Chlorine is the safe default: every manual we read accepts sodium dichlor. Bromine depends on the maker. Hot Spring accepts one-step granular bromine but bans compressed bromine and BCDMH tablets. Master Spas accepts bromine tablets but says not to use them with ozone, UV or ionizers, with its EcoPur filters, or with its Comfort Floor mats. Bullfrog gives a bromine range of 3 to 6 ppm, Sundance 2 to 4 ppm. Check what water care your tub has before choosing; see hot tub ozone vs UV.

Plan on a water change. Bullfrog's manual says owners moving from bromine to its chlorine-based system must drain and refill first, while moving from dichlor needs no drain if cyanuric acid is low enough. See how to drain and refill a hot tub.