Water heater sacrificial anode metal selection
I go with zinc because aluminum swells more and when aluminum breaks down it can plug flow restrictor and ariators.  you will need the model number of the water heater to get the correct length. I find solid rods last longer I have segmented break at the joints several times.
 In 1 home I installed an electronic annode it was very expensive almost 200$ but that home had very soft water and standard annodess were almost gone in 1 year I never had to replace the electronic one and it was still going strong after 10 years.