As to whether J is worth it, that's quite a subjective question. It's a long flight in Y, a touch over 2,000 miles, so you might appreciate some extra comfort.

For CX v KA, the scheduled aircraft looks like it will be a 773 on CX and A333 on KA. They have very similar business class products, see:
http://www.dragonair.com/ka/en_HK/tr...ess-class.html
and
http://www.cathaypacific.com/cx/en_H...ess-class.html

Four points to note though:
- CX often substitutes long-haul equipment onto this route (in the last month I see it has been operated by 744s, A343s and 77Ws). So if you choose CX you open yourself to the possibility of a nicer seat
- But that said, KA have in the past sent their A333 with First Class to Bali, and 8 lucky J passengers have moved forwards. That looks to be rarer than CX using long-haul equipment though.
- KA food is better (in my opinion)
- If loads are light, the KA flight is the first to be cancelled.