If you can readily make 1K, fly to your favorite leisure destination Australia several times a year, and do it from NYC instead of LHR, then United sounds really appealing. Wouldn't it be great to often fly such a long distance upgraded for free to business class? United also has had much better mileage promotions than AA lately in my opinion.

Otherwise, I'd pick AA for the reasons others have mentioned, but then I'm biased in favor of them. As mid/top tier, I did get to use the Qantas lounges on my recent trip around Australia, they sure were great.