Is JL or CX better in J? I have tried CX J which I liked. I am using AA miles. Which has better availability, seats, service?

CX is lie flat 4 across. JL is slanted flat 7 across. No contest.

It might be helpful if you gave us a little more information like where you plan on departing from and where you would like to go. JL has a few different seating configurations so having that information will help us give you better advice.

lax-bkk/icn/tpe.

From LAX JL flies their newest SS7 product, which is staggered and allows for aisle access for all seats. Many also prefer the food on JL to CX (especially for Japanese and Western food), but out of LAX it will probably be a wash. CX has reverse herringbone seats on most (all?) of their long-haul flights, including to LAX.

CX will get you closer to your ultimate destination if you're traveling to BKK or TPE; JL is better for flying to ICN. With JL you're very likely to have a regional configuration, while CX will have a mix of both long- and short-haul aircraft.

If you've tried CX, and there is availability in JL J, I would try JL so you can compare them both.

CX is a 1-2-1 configuration, which works well in my experience both for single people (in a window seat) and couples (in the two center seats). JL is in the process of upgrading their J product. The old J is paired angle-flat that are very outdated. The new J is innovative, with side aisles providing direct aisle access from each seat. The new JL J seats have more privacy than CX's J, with a partition that can be lowered for two people traveling together.

I think JL has a larger video screen.

The CX seat feels more spacious, while the JL seat feels more private. In CX, you see other passengers while seated, while on JL you don't see them (nor they you).

JL has more usable space and storage space. Some CX J seats have an open area under the ottoman, but it can't be used during taxi, takeoff, or landing. Most seats are closed under the ottoman. Whereas JL has a shelf and more storage. (The shelf is on front of you in a window seat, and along side you in an aisle seat, but the aisle seat has part of its shelf area taken by the video controller of the window seat.)

Service is excellent for both. Food preference is very personal; some people will prefer food on JL while others that on CX.

If it's a choice between CX J and JL old J, go with CX no question. If the choice is between CX J and JL new J, I'd suggest trying one each way if you can.

It really comes down to the amount of privacy you want, and your preference for Japanese vs. Cantonese/Hong Hong inspired food (that is still airplane food).

Western-style food-wise, JL has better offerings, and a better snack menu.

Do you even travel?

Never.

I assume the OP means long haul wide body flights. In contrast, JL do operate 738s on some Regional routes (eg NRT-TPE). In this case, I'd take CX's competing 330 in a heartbeat.

I would take CX personally. Whilst I have never flown JL's new J product, the CX one is excellent. Vary spacious, private, fairly good food for an airline. Also, the bed is very comfortable. Their 'suite' is comparable to many airlines' F products. In fact, I had the option to fly BA F or CX J HKG-LHR 4 months ago and readily took CX.

It's quite difficult to make a comparison with zero knowledge of one of the two being compared. You might want to read some of the posts above yours, which discuss the advantages and disadvantages of each.

While BA F is nowhere near as nice as CX F, it's still much nicer than CX J. Since you don't say that your choice was influenced by cost or other considerations, I'm surprised that you would make this choice, although if you are making decisions and recommendations to others without knowledge of the alternatives, I suppose it isn't surprising.

And I have...

As stated quite clearly, I have not flown the new JL product but have experienced their soft-service inflight and on the ground. CX have a very good product, which in my opinion (and I would assume a few others) would rival other's F product (hence the reference to BA). Therefore, suggesting to the OP that the CX experience is very hard to beat.

In the essence of transparency to my BA reference, cost was not a factor. It was a one-way fare and CX J was approximately NZD$3,500.00 more than BA F (CW wasn't available). Having flown BA F and CX J/F several times, I chose CX J (F wasn't available, it wasn't me paying for the ticket, and I have no loyalty to any airline other than NZ). With some of the work I do, it is imperative that I travel as comfortably as possible and BA F doesn't do it for me.

End of line.

I'm surprised that CX J was more expensive than BA F, and that you prefer it, but I am happy that you were able to travel in the manor you find most comfortable. The "YMMV" expression is very true.