I saw research somewhere that said mag sulphate is excreted very quickly by the kidneys compared to mag chloride. Can't remember where I saw it. At the time I was googling all I could on mag chloride.
The stability constant of mag chloride is 0 (zero) which indicates it changes totally to its ionic form in a wide range of pH's. I couldn't find the stability constant of mag sulphate. So most likely, the chloride form would work better for transdermal application.