One thing missing (I believe) from all the above is that for the flight from the us, flying with AF, kl or lh means you benefit from ec261 protection and flying dl or ua means you don’t. If you suffer from cancellations or bad delays due to weather or anything outside of airline control, Eu airlines will have offer you accommodation, subsistence, etc, us airlines will tell you to deal with your travel insurance, on the flight from Europe you’ll be protected in all cases.