As far as the value of miles go, AA's are more valuable. UA has recently increased their award chart so awards are more expensive. In the off-season, I can get to Europe in coach for 40K miles. UA would cost me 55K. AA has nice OW awards where you pay according the miles you fly, and it also has one-way awards, which can be nice, especially if you are into the MR game.

But, to be fair, UA has a better presence in Europe. For many places in Europe, I have to fly AA to London and transfer to BA. AA is pretty bad in Eastern Europe. It's good in South America. UA is better in Asia.

As far as upgrades-and *sniff* I don't have much first-hand knowledge of this, neither will give you complimentary upgrades that aren't op-ups for mid-tier. You have to earn stickers. Since both AA and UA use ORD extensively, I wouldn't expect many op-ups, either. Though I think AA usually has a bigger first class cabin, so your chances of getting into first class-using stickers and otherwise, are greater. But if you don't land in first, then UA at least has E+, which gives you more leg room. (Assuming that you aren't quick enough to grab a bulkhead or exit row).

I think AA just runs smoother. I've had problems with miles crediting to UA, UA seems to cancel more because of weather (probably because their two of their hubs are in DEN and ORD) and AA seems to let people chose their seats more often.

I don't know if you'll join a lounge. I have no idea what those are like where you're going.

I'm sure others can talk more knowledgably than me, but this is what I've gathered.