We did a lot of research before our trip last year. I highly recommend choosing an itinerary that includes Glacier Bay. Yes, "but there are other glaciers", and yes "but some people like some of those other glaciers better", and yes "but it isn't all about glaciers", and (and so on and so on and so on). Do yourself a favor and put those "buts" aside. Unless you plan to cruise to Alaska a number of times in the years to come, you owe it to yourself to make the most of your experience and include Glacier Bay.
Including Glacier Bay means either choosing a pricey smaller cruise line, or choosing Holland America or Princess, or NCL. (There are a few Carnival itineraries that include Glacier Bay, but so few that it isn't worth wasting time trying to fit one into your schedule.)
I also highly recommend choosing an itinerary that embarks and/or disembarks in Vancouver BC. It is a little more expensive, and a little more difficult to fly to, but generally those cruises cruise between Vancouver Island and the mainland, which is incredibly scenic, while itineraries that embark and/or disembark in Seattle cruise up to/down from Alaska in the open ocean.