I don't have any experience at all with NW or CO in the past 10 years, probably mostly because this low-revenue frequent flyer (Premier most years, made PremEx last year by the skin of my teeth) has been completely pleased by my experience with United as a leisure traveller. Access to Economy plus seating is a huge perk for making even the lowest-level elite status (which I can do at even the cheapest fares, unlike DL), and since I fly mostly at times when the real road warriors aren't flying, I've had good luck with upgrades as well (I generally use all my upgrade certs). Flying widebodies domestically (with their big premium cabins to increase my upgrade chances) is nice, too...
We flew AA from SFO->DFW->MIA->Quito and back this month, and based on my limited sampling, I would tend to say that on the mainline equipment, UA has better seats, better inflight entertainment, and, of course Channel 9... However, MRTC makes AA my second choice.
Now if it wasn't for that bankruptcy thing...
Dan