United doesn't do challenges. American does. There's a wealth of information on the AA forum about challenges if you'll do a quick search there. A platinum challenge will get you to midtier and double miles on everything.

It looks like AA only has a dozen flights a day from your gateway airport, so that's something you might want to look at first.

The top tier program at AA (100K) offers 8 VIP's each year (plus 8 when you initially qualify), and you can use those to upgrade international routes if space is available. I've had a 100% success rate on every upgrade I've asked for since moving to AA 21 months back, but others report they are not as successful, or have to wait and clear at the gate. I book months out though when there is probably better availability.

No regrets making it to top tier on AA. I have held on to midtier at UA primarily for their Australia awards (90K in business versus 125K in business using AA miles on Qantas). That might be something you should look at, too.