FCO is long enough to get decent sleep, but generally I always opt for the later departure unless I need to arrive earlier, e.g., for work or for onward travel. But as mentioned there's no difference in on-board service.
+10k for F is a no-brainer in my book - you can have a real meal in the Chelsea lounge before departure. The 77W J seat is more private than the F seat, but the F seat is bigger, your feet aren't in a cubby (even though it's not tooo bad in J on this configuration) and so it's just more comfortable.

Also, I have vowed never to visit Italy in the summer again (maybe up North).