My 2p worth is that the food and drink offering in BA F is not that great and certainly not a huge leap from J. Compared to the latest J seat the IFE in F is also poorer (i.e. smaller screen and requiring an adapter to power laptops etc.). Service can also be hit and miss, depending whether the "good crew" or "bad crew" turn up to man your flight.

That said it depends how you normally travel, if J or F is a "treat" rather than your normal class of travel you will probably be quite happy with J. For me it is a no brainer, I would always choose F on a redemption or where the cost premium to J is not enormous, because the F seat is significantly better than the J seat. Beware however the tattiness of the F cabins. On a recent outbound flight to HKG I had no lights for my seat (1A) so had to sit in darkness (apart from the LED "reading" light) after the main cabin lights were turned off. On my return flight only the more diffuse of the 2 personal lights were working for both my seat and my wifes. It is also a fairly common occurence to find bits of the F seat falling off.

I would go against conventional wisdom and recommend you choose F for the sleeping journey, the service in F is not that great when you are awake and since the IFE is better in J it may be more enjoyable. You will definitely benefit from a much more comfortable bed in F when sleeping however.