CX J is pretty good especially with their new seats.

The main difference between J & F all comes down to the details like:
1) More space in the seat itself
2) Comforter and down mattress pad set by attendant for sleeping
3) Pyjamas
4) Food comes separately plated in F, in J they come in a tray
5) When departing from HK, the CX F lounges have an edge such as private cabanas and ordering from a menu for meals. Not much difference between F and J lounges outside HK.

Either way, both are amongst the best in the industry for their respective classes so you'll still enjoy the journey!