if you are willing to pony up the money for economy comfort, I would go with Delta, as its slightly more leg room and most delta long haul flights have the personal AVOD. United economy plus only provides additional leg room if its on a 747. Korean air doesnt have an economy plus section, but their "regular" economy seats are larger than the others (34' pitch vs 31 or 32). This is assuming that prices are relatively similar. Also, service on Korean Air is hands down above and beyong what Delta and United can provided.