I fly to HKG alot and I did briefly have CX elite status, I found AA's frequent flyer program much much better for my needs and I now have elite status with AA. Two rtn transpacific flights to the east coast should get you enough EQM's for the lowest elite status tier with One World, which is Ruby. You need to fly 30,000 EQM's on CX to qualify for this tier, or 25, 000 EQMS with AA. Flying across the Pacific 3 or 4 times a year should give you enough EQMs for a higher tier, Sapphire. To qualify for Sapphire with CX you need to fly 60,000 EQMs, 50,000 EQMs with AA will do the trick. With AA you can also qualify faster on points, if you fly in premium cabins.

Only the higher CX economy fares qualify to earn miles on AA. If you'll be buying the lower fares on CX while you bank miles into a OW airline's FF program, you will need to deposit these miles into CX.

Once you have earned elite status, with either airline there are some differences you need to be aware of, CX does not give bonus miles to elite members, AA gives 25% or 100% bonus miles depending on what elite tier you have. AA has electronic upgrades for domestic flights that you earn in addition to the mileage you earn. These can be used for domestic flights even on the cheapest airfares if space is available. With CX the only way to upgrade is using miles, on the more expensive economy airfares.

Upgrading a transpacific flight with CX using miles doesn't cost $, with AA you need to pay with miles and $ for the upgrade.

The AA you have to connect in NRT (I've never connected throught PVG or PEK), so your HKG - NRT sector will be on CX or JL. AA flies from NRT to LAX, JFK, DFW and ORD. CX has non stop flights across the Pacific to SFO, LAX, JFK, YVR and YYZ.

If you are going to make same day connections to cities in the US that could have infrequent service, check connections, sometimes, this may not be possible when travelling CX for the transpacific sector because of flight times.

Regardless, of whether you have elite status or not, CX miles have an expiry date, so you may lose some miles if you haven't used them by the expiry date, with AA the expiry date of your miles are "renewed" everytime you have activity in your account.

Now, after all that, why not continue to fly CO and UA? Both fly to HKG, you already have elite status with them. UA also has a hub in NRT, so this gives you the option on flying transpacific via NRT or directly from HKG. With AA and CX, you may not have this flexiblilty depending on which FF program you choose to join (remember, the cheapest CX fares do not earn AA miles and AA miles can't be used to upgrade on CX).

Just wondering why you'd rather fly a OW airline, when you have elite status with 2 Star Alliance airlines and both fly transpacific to HKG.