we took the bus from vancouver to seattle last summer...it's okay. they do stop at one rest stop about 1/2 way through & there's a bathroom on the bus. (our bathroom was quite stinky & some of the other passengers complained.) the seats were comfortable. we got an older 'motorcoach'...there were some shiny, more modern ones, too.
not sure how much time you have, but we rented a car from seattle to vancouver. for the day it was about $80. that was nice because we took our time, ate when we got hungry & stopped when we wanted. the border crossing did take about 2 hours, but peace park is beautiful. (fyi. on the bus on the way back, the border crossing was only about 10 minutes...but at a different location.)
we did it for the same reason...saved about $300/person on airfare.
hope this helps. have fun!