I am looking to do the same thing (Australia in 1-2 years, probably from the Midwest). I chose UA over AA (with some concern due to C.11 of course, but AA'll probably join them soon :-) ), mostly because it's a slightly lower award level for F to Australia.

I have been second-guessing my choice a little bit, but I think that the ease of earning makes a big difference. If you end up concentrating your travel on an airline/partner combo, make sure they have a good enough frequency of service from DTW to make it worthwhile. For instance, UA is the 2nd dominant in MKE behind YX. And I get 500 miles for each 67 mile jump to ORD. Lots better than flying YX direct, or DL to CVG/ATL.