Either, as it's the same chemical. I use solid, but I imagine that liquid is easier. Mainly because solid needs careful storage - it has a tendency to draw moisture out of the air and then set into a solid mass. Magnesium chloride does this also (so does common salt, sodium chloride, but not as aggressively). Also, I imagine that liquid would be a little easier to use, both in terms of measuring out a fairly precise quantity, and getting it into solution in your mash or wort. I didn't actually know that you could buy it in solution, I might try it next time I need some more.