having worked in both the heat exchanger world and now in the yacht world. we only used zinc for a variety of reasons
you dont hear of an aluminum anode pencil in a heat exchanger. if you walked into any of the large yacht builders and said hey, lets hang aluminum on the keel vs zinc, they would all toss you in the drink.
the Aluminum anodes came out because people complained about having to replace the zinc anodes every 6 months or having to sand off the aluminum hydroxide coating that forms on the zinc when having in fresh water prior.
pure aluminum ends up with an oxide layer that protects the anode. So Aluminum must be blended with zinc, iron, and indium (navalloy) or even lithium blends to have enough precipitation to actually continue to work. additionally the higher output of aluminum anodes causes localized embrittlement in steel structures, so now the rage is low voltage output aluminum anodes where they mix in copper
the aluminum/zinc/copper blends and the aluminum/zinc/lithium are a joke as the aluminum and copper or aluminum and lithium become its own battery source in the presence of calcium and sodium and the protection goes out the window as you can call one energizer and one eveready.
and yes, aluminum anodes are lighter than zinc. material density would indicate that. why use it as a marketing ploy. we size zincs (anodes) by their square area, not by weight.
also be wary of low cost aluminum anodes as they are not proper anodes. they are generally low-cost off--shore knock-offs that do not have the proper blend of aluminum/zinc/indium or aluminum/zinc/lithium to actually work as an anode.