Which airport?

I'd choose Amsterdam, hands down. Amsterdam is a very user-friendly airport and I think it is much better for connections than JFK. When I have to connect, I always prefer to connect in Europe over a city in the U.S. If your first leg is delayed and you are changing in Europe, you usually can find a flight to your final destination on the same day. If your flight into JFK is late and you miss your connection, you may not be able to get another flight to Spain until the next day.

Laura nailed it with her advice.

The only arguments for JFK, as far as I can see, are if the timing of the Amsterdam flight would make it harder to nap on the plane. E.g., if the Amsterdam flight arrives at 7 AM versus Spain flight lands at 10.

TC, I'd also highly recommend changing in Amsterdam. It's a single terminal airport (although very large!) and with the shops, etc. there, it can be an interesting place to spend layover time. One thing about Amsterdam that I find interesting is that after the plane lands, it takes some time (15-20 minutes?) just to get to the Terminal. Happy travels!

Amsterdam, for sure. And the staff speak English as well as or better than many Americans speak English.

I agree with everyone that you should avoid JFK at all cost. We once flew LA to JFK to Heathrow to Stockholm. What a nightmare! The flight out of LA was delayed due to thunderstorms. We arrived at JFK with eleven mins to spare to catch the Heathrow flight. The gate attendent kindly held the plane for us. They were paging us to get to the gate ASAP. We ran downstairs, across the terminals, back up the stairs to the new terminal. Got confused with the signs and almost ran out of the airport. Ran back in. We made the flight but not our luggage. After a week in Sweden, my luggage arrived the morning I was leaving.
By the way, avoid Heathrow too. That's another story.

Your thoughts and insights have been most helpful. Amsterdam it is. Many thanks everyone. TC