From emailing the owner this is what I was told. hard water formula will have less cal mag because it will be provided by the water more ppm in the water means more minerals are in it. The soft water formula has more cal mag in it since the water is low ppm so less minerals are in it.Now im confused...
RO/SOFT is great for all hydroponic applications such as rock wool and coco. It's specially formulated for soft water (under 200PPMS or .1-.3 EC).
TAP/HARD is great for all hydroponic applications such as rock wool and grow stones. Being a more acidic formula, specifically for neutralizing alkaline water supply, (175+PPMS or .3-.7 EC).
If I were you I would get the hard water and a small bottle of cal mag. If you see cal/mag deficiencies you can add the cal mag. It's easier to add than take away in my opinion.