Dude, United is awesome! I am a frequent flyer with both United and Northwest. I have no preference for one or the other but when I book a flight, I silently hope for United having the lower fares (simply because of Channel 9).
I don't think you're selling out NW by flying UAL. UAL has a wide variety of aircraft that you can experience so if I were you, I'd get on as many of them as possible. Also since you are flying United for the first time, definitely enroll yourslef in Mileage Plus. it's a great program. But, your miles do expire with United if there is no activity in your status within three years. I know you may have been used to NW's miles which don't expire but UAL is a little different. Also, if you have status with WorldPerks, and you tell United that, they will match your WorldPerks status for MIleage Plus - but only for a year. You have to fly the rest of the miles yourself.
Anyway, enjoy your flight, don't forget to listen to Channel 9 and post your experiences on the trip report forum. Love to hear about them.
By the way, the flight from SFO-ORD doesn't happen to be Flight 141, does it? If it is, that's the flight I take most of the time when I come back to ORD from SFO. The ORD-SFO and back route is a great route, especially if you land on Runway 27 where you start your approach headed East towards Lake Michigan (opposite from the airport) make the 180 degree turn in middle of Lake Michigan and head straight west to the airport. Even better in a 777!
Enjoy your flight.
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!