I don't know if it really matters all that much which brand and which type. At least for most people. People with atrial fibrillation have a special need for magnesium.
It's actually not that easy test magnesium levels, the best type of test is an EXA test which test tissue levels. But it cost $300 and I doubt I will ever get one.
It's true MgO2 isn't absorbed as well, but there is a lot more mg per pill so you still get end up getting more Mg by the fact that there is so much more magnesium.
I like this explanation of absorption.
Magnesium Oxide Delivers More Magnesium with Far Fewer Pills
However, there is one form of Magnesium that is different that is Magnesium L Threonate Capsules (Magtein), because this is the only form that can cross the blood brain barrier and has been studied.
Magnesium L-threonate prevents and restores memory deficits associated with neuropathic pain by inhibition of TNF-α.
Magnesium L-threonate prevents and restores memory deficits associated with neuropathic pain by inhibition of TNF-α. - PubMed - NCBI
I have tried many forms, currently I take a tea spoon of bulk supplement's Magnesium Citrate, 2 caps of Magnesium L Threonate and 1/2 Puritan's Pride Triple Magnesium complex (three forms of magnesium.).
Right now it's just a matter of using the supplies I have, when I do I will order plain old Magnesium Oxide and take a couple of capsules of Magnesium L Threonate (Magtein).
I get Magtein from either Now or Source Naturals. They recommend 3 caps, it's a little expensive so I take 2 since I also take the other types of magnesium, I figure I get more than enough.
People with atrial fibrillation often take Remag, it's $60 per 16 oz and ends up costing about $20 per month. Not something I need, but it's likely the best.