I think you will find the 7 day round-trip itineraries pretty much the same whether you leave from Vancouver or Seattle. You don't say where you are coming from; but if you are flying I think you will find the air fares much less to Seattle. Also, the distance from the airport to the pier is less in Seattle. I can't really think of any advantage to going out of Vancouver.
I agree with Penny: Princess is my favorite cruise line because of its quality and value. They offer Anytime Dining and Princess ships are elegant and more refined; definitely not "party" cruises.
You sound like Princess people to me. But do as Penny suggests: see a good cruise travel agent. They can present all the options available and help you make the best decision.