Welcome to Cruise Chat. We have done one 7 day RT from Seattle and 2 from Vancouver. One major difference is the foreign port requirement. Since you leave from Canada you do not have to have a foreign port. The trip from Seattle make a late port stop in Victoria. It is a very nice port, but a very short and late stop.
Leaving from Vancouver gives you the same ports, Juneau, Skagway, Ketchikan and scenic cruising in Tracy's Arm (at least that is what ours did). What you gain is a scenic cruise day in Glacier Bay. That is a great trade IMHO.
Also you cruise on the east side of Vancouver Island, not the west. The west can be a little rougher. The East side is more scenic, but most of it is done at night anyway.
Either one will give you a great cruise. If you have an extra day, both Seattle, and Vancouver are great places to visit.