Where do you want to start from? If in Texas, then DFW would be the logical choice, if in California, then LAX
The pro of LAX is that it is a shorter flight and less chance of bad weather. Where are you actually starting from and what is the duration of any connecting flight
There is no way that I would do IAH-MAN-SIN-SYD over DFW-SYD -. Fly 4700 miles to Manchester to end up further away from your destination - plus nearly doubling journey distance just to go in business class seems insane