Both QuickShuttle and Greyhound can have that waiting problem at that border in both directions. I'd be curious as to what they say their maximum wait is.

On a pre-9/11 trip, the QuickShuttle bus driver, when crossing the border into the US, tried to see if anyone had an airplane connection at SeaTac (or was willing to say they did) because apparently that could help speed customs along. I hope he wouldn't recommend that now.

The last trip I went on with Greyhound from Seattle involved a 1/2-hour wait crossing into Canada, when a student from Colombia was vigorously questioned.

Grayline shuttle from SeaTac to downtown Seattle + Greyhound to Vancouver can also work.