I was in the same situation last year when health kept me from flying until late October - leaving no chance to qualify for EXP on AA. I looked at all of the options and chose BA, mainly because their system allowed me to qualify for Silver on one RTW and they delivered a free club membership with that level. I found that BA was far better over the Atlantic that AA in Business and will be using them in the future.

In Perth (where I lived for 8 years, by the way) the options may be different. It would depend on where you want to fly when using your points as well as how fast you can qualify for elite status. Show CX how often you fly and they might give you an immediate elite status.

AA is OK for collecting miles, but as I recall you need 4 domestic segments each year. I think BA is better than AA, but you don't get bonus miles as fast.