I believe this issue has been discussed in the forum in the past, but here goes anyway:
The 60mm is a more versatile lens, as you can both shoot macro and fish portrait, but you must get extremely close to tiny subjects to get a good reproduction ratio, which makes lighting a bit tricky.
The 105mm is superior to the 60mm when the subjects are shy, as it offers the same reproduction ratio as the 60mm at a greater working distance. On the other hand, it's harder to get a shot of anything sizeable because of the longer distance required to the subject.
Others will probably add to this (e.g. DOF issues), but these are the fundamental differences.