So far we have been advised not to take tours offered by cruise line. You don't get to see enough with a crowd of 30 on a bus. We have been given repeated recommednations for tour guides in many cities. We (two) are looking for others interested in booking with us.
Private Shore Excursions are the way to go in the Med with large tour buses not practical in many of the ports. We used several and would definately go down that route again.