If you can, definitely go for F on the JFK-LAX flights. If you're only in J for the flight then I'd still pick a 762 J seat over a 738 F seat. While I'm a big fan of the 738 F seat, it doesn't have the extendable legrest that the 762 J seat has. I could be very comfortable in a 762 J seat for the transcon. I'm sure the 762F is a notch above that.