you should stick with the airline that gives you the most FF mile earning opportunities.

And that is American. United has the best alliance system, with the most airlines to your disposal to cash in your miles.

I suggest you get a AMEX Starpoints card & start building miles there to transfer miles to airlines where you have accounts with so you can fly where you want.

Persoanally, in the future, I recommend earning miles & cashing them in the Continental Airlines program. They have the BEST mileage redemption program (i.e. 40/50k miles for USA-Europe vs. 40/60k or 50k for American & United respectively) for infrequent fliers AND great service compared to the rest.

Of course, you being Aussie, Quantas is most likely the airline you use domestically so you're probably forced to stick to Quantas--which is still a good choice.

Regardless of your decision, just choose 1 airline in the future to concentrate your miles. You'll derive the most benefit this way!