What's the difference between magnesium oxide and magnesium gluconate?

More mag in Mg oxide: Mag gluconate has less magnesium on weight to weight basis i.e. On milligram to milligram basis. But the product you buy may tell you how much of magnesium on it. Sometimes they specify "contains however many milligrams of mangesium". Clinically, there is not much of a difference since all magnesium products over the counter are of low dose since if much will cause diarrhea and possible arrhythmia.

Not much: Both are good magnesium suplements

Absorption: Magnesium gluconate is likely better for you. The studies that show good absorption of magnesium oxide were done on rats. Human studies do not show great absorption of magnesium oxide. This being said, oral magnesium in general is not all that well absorbed, which is why too much tends to cause diarrhea.