JHCCoffee I currently use a BWT water jug for my v60 coffee.
Me too. Alkalinity of my filtered water measures around 70mg/L which is fine for me, I find alkalinity of less than 40mg/L can introduce generic sour notes.
I also use a Zerowater jug, mixed with tap water to achieve around 50mg/L alkalinity. I have no preference between Zero & BWT, but BWT is less faff (though mixing Zero Water & tap is easy with scales at a ratio of 1 part tap to 4 parts Zero with my water) and less things to wash up (container for storing mixed water, rather than leave in filter jug).
I don’t think anyone knows what the impact of calcium vs magnesium is for hardness in brew water.
Appropriate brew water (ball park, for me is 40-80mg/L alkalinity) with good brewing protocol & coffee/origin/roast tastes best. The latter two will still be better with non-optimal water, as water does not affect measurable extraction (cannot fix malfunctions like under-extraction, or grinding too fine) and cannot make coffee you don’t like taste great. Get these factors nailed down first with a basically reasonable water - method & coffees/origins/roasts, then fine tune the water.
Too much alkalinity will give more muddled flavours & heavier body to the coffee brewed.
ZeroWater is better than BWT if you are wanting to make/control all aspects of hardness & alkalinity.