I began my frequent-flying career in Illinois as well. Over the years, I've found AA and UA to be fairly similar in terms of their FF programs - more similar to each other over the years in terms of rules, status tiers, upgrading, etc. than either has been to DL, NW, CO, or US.

Now that there are essentially three mega-programs, DL, UA, and AA, I still find UA and AA to be very similar on the major points.

I'd first make a list of your desired int'l destinations. You may find that one alliance (Star or Oneworld) covers your own route map better than the other. For example, someone who wants to fly to Germany often is probably better off on United because you can also use a fairly extensive US-Germany service on Lufthansa.

I'd also look at your domestic travels and see if UA or AA covers those destinations particularly well. Don't forget to consider Continental in all of this...they've merged, but some sources may not readily show all of the CO routes as United yet. You may end up deciding to move some medium-haul domestic travel to AA/UA and just leave the short stuff to WN. (That's where WN is the strongest.)

For me, I know I'd probably lean a bit towards UA in the long run. I like Star Alliance better than Oneworld, I like Economy Plus when I'm on UA metal, I like the fact that UA has west, mid, and east hubs, and the addition of the entire CO route structure just adds to the overall coverage.

There are slight variances in the mileage award charts...for me, the big win was when UA and AA both implemented true (no-bull) one-way awards on their own metal. That right there makes their miles more valuable than DL or US miles. (Next week, I'm flying to Canada...up on AA, back on UA, 12.5k miles each way for a ticket that would have been $650 R/T.)

AA gets a couple important nods: ability to make million miler on all earned miles and (I believe) better ability to use one-ways deeper into the Oneworld network, beyond AA metal.