A number of people have mentioned the Star Alliance as a reason to choose United over Delta.
If Star covers many routes unavailable from Delta's Air France/AeroMexico partners, this is certainly a reason to choose United. However, if the routes overlap Delta is the way to go because elite Delta status earns the appropriate mileage bonus on Air France and AeroMexico. UAL premiers earn no such bonus on partner flights.
I am United Premier member (which will not last) but can say that my level does nothing for me when flying Air Canada. If I were a premier executive it might be different, but I am not (nor is the flyer who launched this discussion.) It's that the special elite lines at YYZ which whisk you through check-in, customs, and immigration are restricted to Star Alliance Gold or higher members. UAL Premiers thus earn no 25% mileage bonus and get to wait on the lengthy (at least in Toronto+Montreal) cattle lines to board USA bound Air Canada flights. I endure this occasionally because otherwise I believe Air Canada to be one of the three best North American Airlines (with Delta and Continental). Helas, I cannot say the same of their partner UAL. Indeed, UAL and Northwestern are the only 2 airlines to rank below average in customer satisfaction for both long and short haul flights according to the latest "Frequent Flyer"/J.D. Power and Associates study
http://www.jdpower.com/releases/Airline051199PP1.gif
Yes, weather and traffic problems beyond UAL's control may contribute to their poor on-time performance. But whatever the reason, Delta is more punctual than UAL. If you have to make a connection in the winter you are generally better off doing it in ATL than in ORD.
Provided two airlines fly where you need to go, you owe it to yourself to choose the more professional one *without* looking at flyer miles.
Otherwise we're all just servants of miles and the industry will continue to placate strike, sick-out, and service victims with mileage promotions. I'd give back many miles if UAL just hadn't lost my luggage and then consistently lied about returning it.