Which has the greater buoyant force on it?

The aluminum has the greater buoyant force on it.

This is because aluminum has low density than lead. And the density of a substance is defined as mass per unit volume. That means density is inversely proportional to the volume of that substance. Hence, a 10 kg piece of it will have a larger volume and due to this, it will also displace more water. So the buoyant force on the aluminum is larger.