Anode rod aluminum vs. magnesium

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is there any difference in an aluminum or magnesium anode rod when installed in a natural gas water heater? In other words is one better than the other? Specially when installed on city water supply, not a well.
The reason I ask is that our local supply house is running low on heaters that have magnesium rods, but have plenty that have aluminum anode rods. I would consider the water quality to be fairly good from our local utility.

Thanks to all

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