If you're quite tall like me, the F seat is longer and a little more roomy around the feet, which makes it more comfortable for me. Also the thin mattress pad makes a small positive difference. You'd also have the ability to eat dinner at FFD in the lounge at JFK and go to sleep immediately on the plane, which is usually my move since the flight is so short. If it were me, I'd do it.

Edit: to add to my comment above, I'm assuming JFK-LHR in F is only GBP 250 more on AA metal, but if there's a BA F seat available for that price I'd take that over AA in a heartbeat. Over many, many redeyes from JFK to LHR, the seat in BA F has consistently proved to be the best for sleep by a noticeable margin in spite of other shortfalls BA's F hard product may have.