The Mercathode system is an “active” impressed current
system. Active systems provide a higher degree of protection than do “passive” sacrificial anodes.
I believe that Mercury
recommends that the Mercathode system be installed, in addition to sacrificial aluminum
or magnesium anodes, on all “Bravo” (& ‘Alpha’?) Sterndrives; hence, it shouldn’t be an either-or question.
See Mercruisers’ Manual Number 11 Corrosion
Guide Part Number 90-17431—4.