Alaska cruises are different when it comes to demographics of the guests. While in the Caribbean your ages would tend to be happier on Carnival, Royal Caribbean, or Norwegian, in Alaska it really doesn't matter as much. Princess and Holland America have the most experience in the Alaska market and are pretty much always #1 and #2 on most lists. But then there are thousands of happy customers each season who went with Carnival, Royal Caribbean, Celebrity, or Norwegian.
The ship is more a hotel and conveyance since Alaska is all about the ports and not so much the cruise ship.
The itineraries for all the cruise lines follow the same paths. Find the itinerary you like and then see which cruise lines are doing it. Then look at the pricing. I think that is the best approach.