Maintainability of natural stone vs porcelain floor tile?
The guy at the store is correct. If the marble is not taking on any water now it is because they are sealed, it is a maintenance item, that will need to be resealed every 5 years at the least with heavy traffic, 7-10 years with light traffic with a penetrating sealer. A kitchen floor is definitely heavy traffic.

The porcelain tile is non porous, just the grout will need to be sealed as a maintenance item, according to the same guidelines as mentioned above.

FWIW, both will chip or crack if something heavy enough drops on it.