Though September is technically the end of the season. Late August often has some of the better weather of the Alaska season; though Alaska weather is unpredictable and will still frequently be cool and wet in August. The scenery is amazing and a cruise ship is the ideal way to see much of it, with stops at many of the Alaskan coastal cities. A round trip out of Vancouver, BC makes for a great trip though it can be harder to get in and out of (more expensive) than doing the round trip out of Seattle.

Holland America and Princess have the rights to sail into Glacier Bay National Park, which is well worth seeing, so I think they have an advantage — but not if they are a lot more. Holland America tends to skew older but that isn't necessarily a bad thing as it is less of a party atmosphere (heavy drinking) of lines like Carnival.

I've used cruisecritic.com forums in the past ( https://boards.cruisecritic.com ), you will want to look not only at the cruise lines for peoples comments and reviews but also the individual ships, as the ships in a line can vary based on how recently they were built/refurbished, etc. Those boards also have great tips for the various ports of call, what some of the better excursions are, best things to do, and how to get them booked or just go exploring in the town on your own.