With UA, you would get:

1) Continued access to UA upgrading
2) Upgrades for miles
3) More miles

But you would lose lounge access on *.

With AC, you would keep your lounge access, and you would get some AC upgrading, but more restricted. And no upgrading for miles.

Since you live in Oz, I figure that lounge access with AN is the best perk you get, and so AC is the way to go.

But, if you are using miles to upgrade UA flights from Oz-USA, then sticking with UA would be better.

Maybe what you need to do is a quick milage run in the fall. Run up to SIN or HKG for a long weekend. That should put you over 50k.