"UA's customer service is the best I've experienced. And I seem to get a lot of upgrades, but as they say, your experience may vary."

United give elite members 4 500 mile upgrade certificates for every 10k miles flown. In an ideal situation, that means that you can be upgraded 20% of the time. However, almost no flights are exactly 500 miles, so an 1100 mile flight is 3 certificates. Also, there are plenty of people who are able to expense $$ for upgrade coupons and so you'll liikely be in a long list of people to be upgraded. This means that even if you do want to use your coupons, you may not get to use them. The chances of you being able to redeem the coupons on the 15% or so of flights that you'll have them for is sketchy.

Also, the coupons expire. So if you can't use them as your status isn't high enough then you won't get to use them at all. Realistically, you might get upgraded 10% ish or the time. I used to fly UA for 3 years at a silver/gold status. I've sat in coach plenty of times while there were seats open in FC. It is purely an insult to their flyers.

AA have seemed alright to me, but I don't think they have the unlimited upgrades that CO and NW offer. Another thing to note is that if you get a lounge membership with UA or AA (as far as I know for AA anyways) you still have to pay the 'nominal fee' (aka full bar price +) and put up with the service with a snarl service from the people in the lounges. It's not good enough that you pay $300 for a lounge pass to a RCC, but you also end up paying $6 for a beer. Try complaining to UA about this. They're run by a different company and you have to actually write them a letter to complain. Rediculous. Personlly, I'd go with the non-direct flights on CO/NW and enjoy the benefits of a company who actually wants your business.