Yes, AA is your only logical choice. BA is the pits for Canadians, and CX really doesn't promote their program here, opting instead to give super deals with AAdvantage. AA will comp you Platinum [equivilent to Gold] and all your miles will be doubled, as well you with earn domestic upgrades from your first flight rather than from the time you qualify after the Plat Challenge. Just call 1 800 486-7523 (from Canada), and you'll be instructed what to do.

Also, if you can route through HKG ex-SYD to Canada you will pick up some pretty signficant route bonuses on CX, and likely be eligible for upgrading to F-class: 10,000 into YVR, 20,000 into YYZ. But also remember you will have to register for these -- and most other -- bonuses on AA.

The only drawback to AAdvantage is that BA transAtlantic flights don't qualify for accumulation, but I see you will be flying AA ex-JFK so it doesn't matter.