Depends on your water source. Magnesium dissolves faster in high mineral water and may give off a sulfur smell. Aluminum will last longer in this situation. If you use a good water source (low in minerals) then the magnesium should work fine.
I think the magnesium ones have a weld spot on the end of the bolt head. So looking at the head of the bolt, in the center, there is a round protrusion sticking out, maybe 1/4 of an inch round.