Personally, living in the Seattle area, I would fly. You just never know what kind of traffic you may encounter, especially when it looks like you are doing this on weekdays. Renting a car would probably cost you more than flying, considering the one way drop off fees.
I took Quick Shuttle up in May, the day before I cruised, so I was fine, but a few on the bus were sailing that day. They made it at 2pm, but I would have been a nervous wreck. And Quick Shuttle does make a few stops. The border was the longest, since there were 2 busses ahead of us, and you have to get off, drag your luggage through customs, and then load back on the bus.