Well in terms of seat pitch (i.e. the amount of legroom one gets in coach), American Airlines currently leads the way in this area. AA offers up to 35 inches of legroom in all of their coach seats, while most airlines have only 30-31 inches on their domestic aircraft. United Airlines also offers seat pitch similar to American's, but there are a limited number of these extended-legroom seats on the aircraft, most of the time reserved for United's higher-fare and frequent-flyer passengers. Unless you plan to use United solely and build up a frequent-flyer account, then I think AA would be the better choice for you in terms of comfort.