Returning to Seattle from Spain

They are about the same as far as number of flights back to Seattle on KLM/Delta.
Search both and consider the cost, duration, and number of connections you have to make.

I agree. Both the airports are easy enough to reach, too.

The one you are closest to and has the cheapest price.

Sounds a bit contrite, but is what it is.

What is your itinerary, or what are the places you are seeing in Spain?

Since you have to change planes to get from Spain to Seattle (no matter where you leave Spain from), it may not be any harder or more expensive to use other airports in the country. For instance, if you're starting or ending in Andalucia, look into flights from Seville (SVQ) or Malaga (AGP). If you want to start or end in Basque country, look at using Bilbao (BIO). Etc.

If you can move your itinerary around, price all the options. When I went from New York in September 2015, it was $80 cheaper to fly into Madrid and out of Barcelona than vice versa - for no clear reason. Since the order didn't matter to me, I naturally picked the cheaper option.

"For instance, if you're starting or ending in Andalucia, look into flights from Seville (SVQ) or Malaga (AGP). If you want to start or end in Basque country, look at using Bilbao (BIO). Etc."

I realize you didn't ask about Malaga, but I'll comment in case someone's reading your post and considering it.

I booked an open jaw Delta flight for us to fly Seattle-to-Madrid, and Malaga-to-Seattle this Fall. I checked the reservation two days ago, and we didn't have our original assigned seats now. I called Delta that night, and he reassigned seats, but said our Malaga-to-Amsterdam portion of the flight was cancelled - WHAT?! He wanted to reassign that portion thru CDG with a 60-minute connection; I know we'd miss the 2nd flight (small plane, local flight -to- large plane international flight). So, I called back yesterday after looking through all of the connection options. This Delta rep told me they couldn't book us on ANY flight out of Malaga because we were using miles to pay, and that connection is now with Europa. Luckily, we have some flexibility, so we're taking the train back to Madrid and flying out one day later.

It might be better to stick with a larger airport (i.e. Barcelona or Madrid). Smaller airports might only have one flight per day that meets your requirements, so if something happens to that flight, you may be in some trouble. Barcelona has a convenient inexpensive bus to the airport that leaves from Placa de Catalunya. The Madrid metro goes to the airport, but it is not always handy to deal with luggage on the metro system.

There are also frequent buses to the Madrid Airport from near (maybe inside?) the Atocha rail station. I, too, find buses more convenient than subways and trains when dealing with luggage. No worries about finding a place to stash the suitcase.