It really depends what is important to you. Cost, the itinary, etc. Are you looking at just a 7day cruise or land and cruise. There is no reason you wouldn't have a good time on Carnival. But for Alaska I recommend HAL & Princess. They have been up there in Alaska longer than the other cruise lines and have a lot of pull in Alaska. They get the most permits to go into Glacier Bay. Some cruise lines don't get any permits to go into Glacier Bay. If you want to see Glacier Bay Carnival only goes in there at the beginning of the season and the end of the season.
Something to consider is the size of the ship you are on. HAL still has several ships in Alaska that carry less than 2000 passengers. When disembarking in the ports you will have less people to contend with. And if the ship would have to tender, then less people would go faster.
If you are planning to do land and cruise, Princess' train comes right up to the ship in Whittier. This will save time.
Just a few things to think about.