what is the advantage of galvanized iron over tin-plated iron?

u are correct

Galvanized iron is nothing but coating of zinc over iron. It is done to prevent rusting of iron. Zinc is higher (more reactive) in the reactivity series compared to iron. Therefore even when there are small damages on the zinc coating the iron does not get corroded as zinc forms an oxide coating in preference to iron. However in the case of tin, it is lower in the reactivity series compared to iron. So if there is a small damage on the tin coating iron gets oxidized in preference to tin and corrodes.