Magnesium Threonate contains very, very little elemental magnesium. It is claimed though not well proved to better cross the blood-brain barrier, but correcting a magnesium deficiency needs to be more than just a (possible) brain tweak for sleep. And it's expensive.

Magnesium glycinate is easy to take (sweet, no stomach issued) and Magnesium citrate is cheap in bulk and easy to add to water.