You have asked the million dollar question! Look at all the magnesium supplements on the web or in a health food store and you will see that no one knows the answer yet.
What I am doing is eating Organic vegetables and I take a multi-mineral blend w/Mag oxide and glycinate and a liquid trace minerals.
During this cleanse I made the mineral broth that both Dr. Anderson and Dr. Schulze recommend. I drink that 3 times a day. Since cleansing depletes our mineral reserves at a fast rate, I
am relying heavily on the mineral broth.