Well, a few observations:

1. Most all UA fares outta the US earn miles. Can't think of any that dont, cept travel agent/non-rev, off-hand.
2. Not all UA cheap fares are upgradable internationally. Those fares that are may be considerably higher than the cheapos. If you get a chance, compare a VIPOW w an SWU. AA may be better here.
3. UA F is quickly disappearing on many US routes (ie. RJ's and TED). But one 'advantage' to you is that MCI has 737's and 757's into ORD or DEN.
4. UA tickets are almost always the most expensive in the market. You can count on that! In fact, United almost has a certain arrogance about it, IMHO! But, that's not to say that UA wont match or have a lower fare than another carrier at any particular moment in time.

5. Historically, UA has lots and lots of mileage earning promos and discount coupons, which can overcome/counter balance a high fare. This is another big advantage to UA. I don't know of any other airline which offers sooo many ways to easily discount a ticket.

6. UA does Asia so much better than AA. OTOH, AA does Central and South America better than UA. But then both carriers all have partners. And you would need to compare the mileage earning oppty's on them. Some low SQ fares don't earn miles at all. So be sure to compare what the partner has to offer in terms of miles as well.

7. If at some point you want to do mileage runs for status, AA's routings are much, much more restrictive than UA's. Thus, UA pays off better, faster.

One of my pet peeves about AA, is that they charge a service fee for award bookings less than 21 days in advance. UA doesn't.

So mebbe you should chose the one whose personality, route and award structure, and elite benefits, matches you best.

All I can say is that Singapore, TG, LH, and ANA FC awards work for me!

Cheers!