If AA is your primary carrier out of AUS, you will probably get 4 segments on many of your flights. If you fly 30-40 times a year, that will easily put you at the EXP level. Once you get there, you will get 8 international upgrades, which are very easy to use, and you get free space-available upgrades for all domestic flights.

I would start with a Plat challenge, though, in order to get double miles (and possible sticker upgrades) until you hit EXP.

I like the SPG card for AA, plus SPG is easy to transfer to other airlines, too. I disliked the AA-branded Citi card.

I don't use lounges much, since I fly out of the DFW hub. AA Admiral's club is cheaper after you get to EXP. There is some sort of AmEx card (maybe Platinum card?) that gets you access to lounges on multiple airlines. It has a hefty annual fee, comparable to a one-year membership to the Admiral's club.