It could be a point or a circle.
If the two spheres are tangent, then the intersection is by definition a point.
Since the two spheres are of unequal diameter, their intersection can't be a sphere (that can only happen if the spheres are of the same diameter and at the same point, then they overlap everywhere).
Now what happens if the two spheres are drawn close to each other a random way ? Well, they will intersect along a circle [see the image attached].
If you want more explanations/..., feel free to ask.
