Cannons vs Railguns

A railgun is more deadly than a cannon firing APDS ammunition. Cannons have 4 types of ammunition to choose from though. Electrothermal cannons have enough advantage in rounds per ton that they can carry as many APDS rounds as a railgun and still have a good amout of the other types as well.

Assume the ETC is used with a laser rangefinder. How many different types of ammunition need to be available to it before the ETC's versitility outweighs the railgun's greater deadlyness?