Rather than reinvent the wheel, here is a very nice pretty wheel
It is very long but very thorough. It deals with all aspects of Mg. For MgCl2 vs MgSO4, scroll to the DIY section
http://reefkeeping.com/issues/2006-07/rhf/index.php
Essentially if you want to keep them in 'balance' you use a ratio of 10 chloride to 1 sulfate. Don't use just sulfate, but you can use just chloride if you do regular water changes.
