Hi Patwice, and welcome to cruise-chat!
Princess has a well-deserved reputation for their Alaska cruises. In fact, Princess is a pioneer in the Alaskan market. But longevity in a market doesn't mean they are always the best. Celebrity offers a good product too. It can really come down to the shipboard experience. There isn't really much difference in itineraries. Princess has personal choice dining, which means you don't have to conform to a set dining room schedule as you would on Celebrity. Given the port intensive nature of Alaskan cruises, I would go with Princess just for the dining freedom.