The NYC-SYD r/t will accrue approx 21920 miles to your AA account plus get you AA Gold Status until Feb 2012 since at 1.5 qualifying points per mile flown, you will attain approximately 30,000 qpoints and only 25,000 are needed for gold status. I am not 100% sure but you may also get an extra 2491 miles if ( as like when a challenge is being performed ) that the flight which you attain Gold status credits with the gold bonus
20,000 AA miles is enough for a r/t anywhere within Australia in economy whilst it takes 36,000 Qantas points for a trip such as SYD-PER r/t
Within the USA , 25,000 points will get a r/t anywhere withinin North America whilst for a r/t , say, to LAX would cost 36,000 Qantas points
Going to Hawaii would cost 35,000 AA points r/t vs 72,000 Qantas points from New York
Getting Gold status will also have benefits when travelling on AA since you will be able to do such things as pre-select exit row seats.
I would recommend sticking with AAdvantage, plus if you can get a Citibank AAdvantage credit card and u have enough points for a free trip to Hawaii with the 15,000 bonus points for getting the card
Dave