American Airlines
For this recent trip to Austin I took American Airlines, mainly because my travel agent told me that they fly regular airplanes (Boeing Super 80s) out of Chicago instead of the tiny regional jets that United uses.
I fly United as a force of habit. When I first started with Mac Publishing LLC, they booked us on United and United gave new frequent flyer accounts a free year or two of "Premier" membership, which granted me access to early boarding and extra room in coach (United calls that front section "Economy Plus"). Those golden days are over, though, and I may switch to American for a couple of simple reasons that show they're paying attention to customers:
a) American gives you six extra inches of legroom throughout coach. Flying coach is still uncomfortable for someone of my size even with the extra room, but it's a hell of a lot less uncomfortable that the deep vein thrombosis-inducing human origami required on most airlines.
b) You can print out your boarding pass from a Mac or PC within 12 hours of your flight, saving you from ever having to go to a skycap or ticket counter when you get to the airport. While this wasn't a big deal for me this time around, because Austin/Bergstrom was pretty quiet, it did shave a few minutes off my entry to the terminal -- and what's more, this could be a lifesaver for those days I'm flying through SFO, LAX, Logan or another hideously busy airport.
The only real differences I noticed in terms of the quality of service was that American's Super 80s don't have any in-flight entertainment -- most of United's jets have TVs or pop-down video screens showing recycled prime-time dreck, and offer you your choice of a few channels of canned music on the in-flight music system. As you can tell, I'm ambivalent about this. Ever since my PowerBook gained a DVD player and iTunes, I've been more interested in listening to or watching what I bring with me anyway. Also, American's a bit skimpy on in-flight meals, too, but airplane food ain't nothin' to write home about under any circumstances.