I would go with the Sapphire/Freedom combo. If you are only considering the Mileage plus explorer card for the free bags and priority boarding, if you will be flying PDX to IAD at least 6 times a year you will be at least the lowest tier elite which will give you up to two free checked bags and priority boarding anyway so I would go with the Freedom/Sapphire for the points bonus, since you are already getting the perks as an elite anyway. Plus I remember hearing somewhere you can transfer your Sapphire points to United miles 1:1 if you prefer.