Depends on your h/w heater. You want the rod to be lower on the galvanic scale than the lining of your hw heater so it deteriorates first, hence "sacrificial" anode.
Some (many?) are ceramic lined, so either will do.
If you have a very bad smell to your hot water, use aluminum, otherwise the magnesium will do just fine.
At $10 every 5 years or so, it's not that expensive either- unless you're full timers who use it all the time.
Gernerally speaking, use magnesium unless you have problems as mentioned, then switch to aluminum. Same advice for home water heaters (I've switched a few to aluminum due to poor water, mostly well water, to eliminate the smells)
