Would you expect to find more impact craters on Io or Callisto? Why?

Callisto. As Io is geologically active body.

Explanation:

Io and Callisto are 2 of the Galilean moons of Jupiter. Io is most geologically active body in the solar system which means that a lot of volcanic activity takes place over there whereas Callisto is not geologically active.

This is the reason that probability of finding an impact crater will be more on Callisto than Io. As the craters might get filled with volcanic eruptions.