Unreinforced concrete will last a lot longer than reinforced concrete. Steel rebar rusts away.

Regarding clay vs modern concrete formulations, my money's on the clay.

Steel reinforcement is only one type. You now have fiberglass reinforced rods or you can use an additive like fiber mesh.

If the steel rusts away, you just have unreinforced concrete.

You have unreinforced concrete riddled with holes, in a shape designed to depend on the steel. It won't last nearly as long as a structure designed with unreinforced concrete in mind.