I personally find that September is a nice time to cruise Alaska. Mr. Ogg is correct that you may encounter more rain and sometimes a bit more chilly, but overall it is an excellent time to cruise. I personally find that Celebrity M-class ships provide a excellent cruise experience, esp. for the more refined cruiser. The service, food, cabins, and the ship itself are superior to some of the other lines. Only Crystal is a cut above from my experience and of course you pay for it.
You will enjoy Alaska regardless of the ship you choose or even the time of year. But, if you truly desire an outstanding cruise vacation, I would book Celebrity's Summit or Infinity.