Your key objectives are priority check in and lounge access combined with low fares.

AC does give you the big advantage of *G for 15,000 fewer points than most other carriers, so if you are no sure you'll hit 50K miles it may make sense.

If you do continue to fly UA, *G does NOT give you access to E+ (unless E- is full, then you'll get priority over others).

One possible point in AC's favour is that if you are travelling back to the U.S., AC typically has pretty aggressive prices to tempt US travelers to travel via Vancouver and they now offer connections on Qantas to get to/from Sydney. They have excellent Business Class service with lie flat seats on 777's from YVR-SYD and as an Elite you can get 2 Special System Wide upgrades (SSWU's) that could be used to upgrade low cost fares from SYD to your US destination and back. Caveat: this route is VERY popular for upgrading so best to travel mid-week and not during peak travel season. As an SE, I trump your E for upgrades.