Flying from Vancouver to Seattle can be expensive (just checking now, a one-day advance booking on Orbitz is $259).
If you do, note that you have to arrive approximately 2 hours before your flight to pre-clear customs & immigration (for most Canada-to-US flying, it's done on the Canadian side, before departure; arrival in Seattle will be like arriving on a domestic flight, no border formalities). The flight itself is only going to be 30-40 minutes long, but factor in taxi times on each end and they're typically blocked at 50 minutes to an hour. From the train station in Vancouver to the airport is about 35-40 minutes on SkyTrain (take their Expo Line to downtown, and transfer to the Canada Line, both run very frequently; but note only half the Canada Line trains go to the airport). Driving/taxi will probably take about as long, and is subject to traffic variabilities depending on the time of day (there aren't any expressways in Vancouver, which is a good thing, but it also means that any trip from downtown to the airport is going to have to stop at every traffic light on the way).
Driving (or taking the bus) from Vancouver to Seattle is, from what I remember, about 3.5 to 4 hours, but is subject to the variability of the border crossing. Taking the bus can be particularly frustrating because, IIRC (unless things have changed recently), there is only room for one bus at a time at the specific border facility (and there are numerous buses, from various companies, operating across the border every day so there's no way of knowing if you'll be the only bus there or not), and when your bus is there, it must wait for everyone to clear before continuing onwards. This means that if one person is having some kind of difficulty getting proper clearance (or if they simply need to fill out more paperwork due to their particular status), you can sit there for a long time. I should clarify that this info is based on my memory of taking that bus a number of times years ago. I probably haven't been on a transborder bus in 8 or 9 years, so it's entirely possible that things have changed since then.
I should also note (in case you didn't know) that, if you do take your original train (and we all hope you do), you should still arrive 30-45 minutes (or more) early, because they do pre-clear at the station in Vancouver just as they do at the airport.