I completely agree with the other posters. CX F is actually on top of my list for F. You will not be disappointed and the differences between 747/777 etc are minor. Take whatever has the best flying time / connection etc.

I have been in JAL F several times. Also a very good product, better than most other airlines in F, even better than SQ in my opinion. The seats are significantly smaller (4 across on the 777 vs. 3 on CX - the CX seat is wide enough for 2 people). I like the CX design where the seats are not completely enclosed, but private enough and removed enough from the aisle for not beeing disturbed at night by people walking thru the aisle.

Both are significantly better than all the american carriers, BA, LH (the 380 seat is ok, the older seats are inferior), LX (their new F seat is nice, but only on the 330). Asiana is ok, ANA is ok, Thai is almost ok, SQ has some horrible seats (on some of their 777, mediocre seats on the 747 and good seats on the newer 777).

In short: You cannot go wrong with either CX and JL, but if you have a choice pick CX.