You're talking routing Europe - Australia via the US, all on United metal? (as would be required for one-way awards)

Normally I'd say that UA's new business is close enough to their first that F isn't that much better, considering that the F amenities, meals, and service really aren't that great on UA.

But for such a long trip I'd say that the more peaceful cabin matters for reduced stress, and F is a better and bigger seat.

And if you've got the miles, and don't see yourself doing F again, then it's worth considering.

Personally I'd save up more miles and do the trip on a carrier other than UA. Or wait until summer when UA will presumably introduce one-way awards with their *A partners. Then you could do F on a proper carrier perhaps [Starnet blocking notwithstanding... note that BKK-SYD availability in F on TG is pretty good.]