If the spheres represent a potassium atom and a potassium ion, which best represents the ion?

If the potassium ions were represented by spheres, the smaller one is the ion and the bigger one is the (neutral) atom.

Generally, metallic atoms (such as potassium) readily form cations (positively-charged ions) by losing electrons in their outer shells. By losing these electrons, the atoms are expected to be smaller in size. However for nonmetals, they readily gain electrons to form anions (negatively-charged ions). By gaining these electrons in their outer orbital shells, the ions generally then becomes larger.