When I read
this post by niner in 2013 I already had tried all different types of oral Mg supplements due to very severe painful-muscle cramps (Magtein too) for years. Since I didn't found much improvement with any of the distinct forms, due to niner's post I also started to use cheap Mg-oxide. And indeed, just the elemental total Magnesium amount supplemented a day would result in a somewhat alleviation of the symptom.
By which I don't want to discourage you to experiment yourself with all the different form, in the end we are all different with different preconditions and bio-chemical individuality. Some really might work better in your case. And different forms are indeed differently tolerated by each of us. For example the 2.5 g/d of elemental Magnesium I tolerate might cause diarrhea already at only a tiny fraction in any other.
Just in my case I could have saved myself a lot of money by using the cheap types right away. In the end only Mg-sulfate infusions I've got since November last year, ceased my muscle-cramps completely