Paine Field / Snohomish County Airport is in Snohomish County, about 25 miles north of Seattle. It is where the Boeing twin-aisle (747/767/777) assembly plant and tour are located. It is off of SR-526 just east of where it joins SR-525 in the town of Mukilteo. Boeing Field / King County International Airport is on the southern edge of Seattle. It is the home of Boeing's flight test center and delivery center for the single-aisle jetliners (737/757), as well as the Museum of Flight. I have travelled to Paine Field from Seattle via Metro Transit (in King County) and Community Transit (in Snohomish County), but it involves changing busses several times. I do not know is Sound Transit, a multi-county system, has any express busses that serve the airport, so renting a car might be the best option.
"So many planes; so little time..."