If money is a factor, flights to- and lodging in- Seattle are usually less expensive. Vancouver is a very nice city; probably nicer for tourists. But I can’t pick one over the other, hands down. The port in Vancouver is more convenient than one of the ports in Seattle, but that’s only about 20 minutes of your entire week.
I find the itinerary and ship (after all costs) to be my determining factor.
If it’s your first AK cruise, I think any set of ports will be fine. (Unless you have a must-do on your list).
I cruised NCL this September from Seattle and did not experience the rough waters of the Pacific. And have not in the past when sailing out of Seattle (maybe we entered the Inside Passage earlier in the journey).
I stayed downtown near the Convention Center and did a lot of walking. Yes, more homeless people than I am used to - but I live in a suburban area. I felt safe as a solo, late middle age, traveler on an afternoon precruise. I do keep aware of my surroundings, but did not feel the need to get out of any area quickly.