As a travel advisor I just got off a seven day Alaskan cruise and have a lot of firsthand destination knowledge as each cruise line is very different. If you are not working with a travel advisor, my contact info is in my bio.
We just returned last week from a 7-day Princess cruise out of Seattle. We did the Holland America one about 4 years ago. Like other posters have stated it is definitely an older crowd on the Holland Amecia cruise. Also, on Holland America the buffet line was shut down in the evening and it remained open on Princess. I highly recommend a room with a balcony. One nice thing about sailing out of Seattle is the Port Valet, where they will take your luggage directly to the airport for you at no cost and you won't have to deal with it until you get to your destination airport