I would go with the Seattle Airport Marriott. Great clean and renovated hotel and it is back off of Int'l Blvd, so it is much quieter. It has everything you need. Your time to the airport even with lights is still quick, but others comments are correct, it is a lot of turns and go north to do south back toward where you started to get into the airport.
Downtown can be fast but the section of I-5 into downtown can be a parking lot of traffic and you could find yourself not getting anywhere for 50+ mins not 20. If you do want to see downtown, take the light rail at the airport right into the heart of downtown - its fast and easy