Both are possible and good. Most of the excursions are very similar. The ship is a bit more expensive due to crew expenses and profit marjin. The major plus of a ship's excursion is that if you return late and its a ship's excursion, they will wait for you before they sail on. In Alaska thats not a huge problem, but in some foreign ports private vendors could have mechanical problems and might return you to the dock a little late. It's the chance you take.