Boatzincs.com: Flexofold prop anodes Aluminum=$19.25 Zinc=$17.50
Fisheries Supply
Aluminum grouper $41.83
Zinc grouper $40.99

So the prices are close enough that other factors are the decider for me. I'll need to find a chemist to explain to me why the aluminum anodes are reputed to last longer.

My diver does not charge for an inspection when the anodes don't need replacement, but charges a significant (but fair) amount when they do need replacement. So it should be a cost-effective strategy to let the grouper hanging on the rail take the full load. I can replace that with no dive charge. Meanwhile the hull and prop zincs should last longer.