UA would be the best for accumulating miles on paid flights. only issue is that they have older planes (and FA!) and stop in sydney. qantas and virgin have direct flights to LAX but the amount of miles you would need on qantas for a free ticket is outrageous and availability in premium classes is terrible.

re; a credit card, SQ is also an option and their miles are more valuable than qantas.

as a reference, on a business class one way ticket to NY, UA charges 67,500, SQ charges 92,750 and QF charges 128,000.

btw, SQ and QF both charge approx. $500 each way in fuel surcharges. United does not.