Don,
We have the Suburban 10 gal water heater on our 2008 3400.
In August 2009 I emailed Suburban on the two anodes - Magnesium and Aluminum and here is what I received for a reply, quoted here in part:
"...All Suburban water heaters are manufactured with a magnesium anode rode. The Magnesium rod provides sufficient protection against a broader geographical range of water types that most RV'ers will encounter in North America."
The email went on to relate that heavy mineral laden water types that are found in the desert southwest and in many well water supplies will prematurely consume the magnesium rod. In that type situation Suburban recommends an aluminum rod as it has a slower consumption rate allowing it to last longer.
So, my conclusion from that was Magnesium anode generally has a better consumption rate and therefore recommended overall.
I order my Magnesium Anode rods from
www.americanrvcompany.com. I am sure they can be found elsewhere with a search.
Hope this helps. Dennis