Hi snilan and welcome to Cruise Chat.
There is always an ongoing debate about purchasing excursions through the cruise line. On the plus side, you know that their tour operators are licensed and bonded, and, most importantly, if there is a delay and a ship's excursion is late in getting back to the ship, the ship will wait for you.
There are tour companies around, and kiosks on the piers of the different stops, that have similar or identical excursions for less money. There are some very reputable companies that do this. There are two issues to consider. First, as stated above, if there is a break down causing a delay in getting passengers back to the ship in time, the ship is under no obligation to wait for you. In fact, the ship doesn't even know you're on an excursion through another company. Secondly, folks not on an excursion purchased on the ship are generally allowed to get off the ship only after all the ship's excursions have been called to debark. Most outside tour companies know this and wait for you, but there is no predicting exactly when you'll get off the ship in each port, and I've heard of tours leaving before you can join them.
So it's a toss up to some people, and a "no brainer" one way or the other to other people. I'm sure folks here will give you their ideas and experiences.