You would have to pay a bit more for E+ on UA, but it sounds like it would be worth it. It's 4 or so more inches of legroom. Otherwise, KLM has non-stops to Amsterdam. Any other carriers, you're looking at connecting somewhere and going through Schengen controls at the connecting airport, then going to what amounts to a "domestic" European flight. It adds hours to the travel process.
Go for the non-stops.