If you have been there before and know the lay of the land, you probably have some choices that you didn't have the first time you cruised there. When we cruise in the Caribbean we rarely take a tour now. Mostly we just walk or ride into town. However, we don't really like taking taxis to go sightseeing. We would just as soon pay a little more and get the advantage of having an organized tour.
Language is another consideration.
If you don't speak the language it is not always easy to get around on your own. This is very important, especially if you want to go out on your own in Europe, Asia, etc. You might have to know when the trains leave, or maybe make some arrangements for a taxi to meet you at the port. In Alaska if you want to do a helicopter tour on your own, you better plan in advance. The cruise lines have reserved most, if not all, of them.