AA has more room everywhere in coach, but flies a ton of super-80s, which decrease your odds of getting a middle seat, yet increase your odds of going deaf from enigne noise.

UA has more room in their front economy plus section, where elite members get priority. On a three-quarters full flight, an elite will often get a roomy E+ seat near the front of the plane, with an empty middle next to them. The United planes tend to be newer and less noisy than American planes. They also run more jumbos (777,747, etc.) on domestic routes (such as west coast to Chicago) United also allows you to earn and redeem FF miles on Delta. [Though not on Delta express - even if it happens to be operated by Skywest, a United Express carrier, but that's another economics thread] They also are starting a code-sharing and FF agreement with US. UA also has headphones on all mainline flights with the cockpit communications available. The cockpit communications can make the flight much more interesting and explain what is happening. [Once I heard ATC chide our pilot for lining up on the wrong runway - right before feeling the plane turn!]

Delta flies to Atlanta.

Northwest flies to MSP and DTW. If you like connecting in the snow, you might like it. The planes tend to be older, but the in flight service has gotten much better recently. It is also really easy to get a ton of bonus miles on NW with frequent promotions.

Continental does a lot of code-sharing with NW and tends to have decent service.

Alaska is a good bet on the west coast. They seem to have some sort of code-share or mileage agreement with everyone under the sun (including AA, NW, CO, and Amtrak in California)

Southwest provides frequent service at low prices. For last minute flights of short distances it is often your best bet. (Why get a prereserved middle seat at the back of the airplane on another airline when you can get a middle seat in the front on WN, or if you show up a bit early a window or aisle)