While I would suggest in order of best food, Celebrity, Holland, and the Princess, which would be great for you, for the younger couple, they would prefer Princess. Celebrity and HAL both attract an older crowd - as I like to say they are for those who appreciate the finer things in life without wanting to pay for them. They offer a more upscale environment with wonderful food, but they cater to an older clientele so there's not alot of amenities for younger folks. Princess is just a small step down, but not a big one, yet they do cater to a younger crowd and have amenities accordingly.
However, with all that said, in this particular situation, at 30,200 gross tons, the Pacific Princess is the smallest of the three ships you're considering, so it will not be loaded full of amenities anyway.
If you want to see Glacier Bay, which is wonderful, then you might also consider some of the newer larger ships both from Vancouver and Seattle that are available.
Check with your agent and they'll be happy to provide more specific information so you can make an informed decision on the best ship and itinerary.