LH gets better in F if it goes from a 747 to an Airbus, because the A340's have solo seats in F whilst the 747 is 2x2 upstairs. If you are travelling alone you may want to ignore LH F for this reason. LH F seat is fully flat and the food is excellent, but the real benefit is the lounge at Frankfurt which is superb, assuming you have time to spend there, which includes a chauffeur service to your plane.