As Marty wisely pointed out the Seattle Link Light Rail is a wonderfully efficient, clean, quick, and seemingly very safe and inexpensive way to get to the downtown Seattle area from the airport.
The only possible negative, depending on your health, is that there is a bit of a hike (short, flat, and always under cover) from the airport terminal to the SEATAC Light Rail station.
If indeed you had health issues that limited your mobility this could make using the Light Rail a bit of a pain. Otherwise, if you have no such limitations, the Light Rail is likely the most optimal way to go.
Take care,
Dan, Tanya, and Hannah on C-Renity