The F lounge in JFK is marginally better than the J lounge (better booze, at least), but some people find it rather gloomy and small. Also, Cathay F (unlike J) passengers can eat at Pre-flight Dining before departure: decent, but not exciting.
For a trip that involves sleep, I find the F seats far more comfortable than the J, with the mattress pad and the better duvet. I don't know that it's so much better that I'd take a less preferable date, but the longer the flight, the more compelling the increased comfort is...