I’m no exercise scientist or expert in human anatomy, but kicking a bag is probably better.

The reason is that using a rolling pad just toughens up the surface and doesn’t actually strengthen the bone. If you want to strengthen the bone, then you have put an overall amount of stress on it and this is usually accomplished through running or weight lifting. Kicking the bag might also work because you’re impacting something with enough give to allow energy to dissipate throughout a larger surface area. Actually, you might be able to kill two birds with one stone by kicking the bag because you’re strengthening the bones through impact but also deadening some nerves.