I've been flying at about the Gold or higher level out of BOS for about 10 years, and from time to time, I've been with CO, NW, US, DL, and UA. I have the most experience with CO, US, and UA. Never really taken AA much, probably 6 AA flights including Eagle in the 10 years.

Right now I am mainly focussed on UA for the following reasons in approximate order of importance:
1. I find that on balance UA recognizes elite status more than some of the other airlines. For instance, they provide hotel accommodations to 1K's even during weather delays, and most agents will help you either through official or unofficial procedures. Most recently I had bad luck with this at US at DCA where I felt that they did not really go above and beyond most of the time even though I was Chairman's Preferred. Some agents did, but most retired or left US Airways. Also as a result of this I often now fly UA BOS to IAD, instead of BOS to DCA on the US Shuttle;
2. Ua's overall in flight service ameneties. Channel 9 for listening to ATC communication on the planes (UA is only airline that offers this), glass cups in first class (US has plastic), and ok food. The UA P.S. transcon flights out of JFK have exceptional in flight service and food. Note, out of BOS, its a valid routing to do BOS/IAD/JFK and then either LAX or SFO in order to get the UA P.S. flight;
3. Over the last 1-2 years UA has had more mileage promotions than US.

Also if your mainly a domestic flyer you might consider CO. At BOS the President's Club, which offers free alcohol and WIFI, is in terminal C outside security near entrance to gate 40-42 area. I bought a lifetime membership when I was a CO flyer so sometimes I will stop in for a drink. Also for just standard domestic midcon flights, CO has perhaps the best food, etc. Even in coach, CO serves meals on flights like BOS-IAH. Also the CO staff has been helpful to me over the years, even when I was a college student.

Also DL has its shiny new terminal A, which unfortunately I had not been air side in yet. One thing that drove me away from DL in the past was its official corporate "Simply Good Business," policy whihc menat no waivers or favors. However, DL has abandon SGB, so who knows how they are now.

Best of luck!