If you don't need the points, definitely fly Y, for the following reasons:

1) J class between HKG-SIN/SIN-HKG has no special fares on CX and it will cost a bomb.
2) It's a short daytime flight. Do you really need the better seat?
3) The business class is pretty crappy on regional aircraft. Someone mentioned that if you choose a flight that is operated by 74A (747-400), then you will get the long-haul J seat. I can say that even if the schedule says 74A, on these short hops, you are never guaranteed anything and CX does last minute swaps frequently. Earlier this week a family member flew HKG-SIN on CX and was subject to last minute swap from 74A to 773.

If you really want to fly J, then consider UA (yes, I know, sacrilege for a OW loyalist). The flight timings are horrible (early, early AM departure from SIN), but you are guaranteed a good long-haul J seat as that flight continues to Chicago ORD and the fare is about half of CX.

Someone else suggested Jetstar. I've flown that and didn't like it one bit. In Hong Kong, they even use bus gates instead of docking at the terminal itself. Certainly, if you're used to travel in J regularly, then Jetstar would be a big downgrade for you. Stick to CX Y if you choose to travel in Y.