What Is The Difference Between A Magnesium And Aluminum Anode Rod And Which One Should I Use In An Electric Water Heater?

For soft water a magnesium anode.  Aluminum is used when you have very hard water or
water that is softened heavily with salts.  Installing an aluminum anode
after you discover your previous anode has deteriorated heavily is
recommended.  If you install a magnesium anode after finding a heavily
deteriorated anode could cause a negative reaction in the water and
cause pressure to release out of the households faucets. If you have to
install an aluminum anode rod, avoid using the hot water to cook with. 
Modern science believes that aluminum in the water can cause Alzheimer's
disease.  Don't consume any hot water. Zinc anodes are rare to find
already installed in a water heater.  Zinc anodes are used to counteract
the effects of sulfur smells in the water.  Zinc anodes are only 10%
actual zinc.  The rest is aluminum.  Do not consume or cook with a zinc
anode any more than an aluminum one.  If the rod bends easily in your
hands, it is aluminum, if not it is magnesium. Anodes have a protective
current of about two feet.  Buy anode rods that are too tall for your
water heater.  Cut them down if you have to.  Try to buy anodes that are
more than 3 feet and 8 inches.