I would absolutely say DL. They are by far the most reliable domestic airline, they have the best customer service, all planes on that route are guaranteed to have TV’s, USB ports, and outlets. The service is excellent on DL, and they’re the best in IRROPS. B6 is simply unreliable, and from what I understand you’re not guaranteed TV’s and outlets on UA or AA. Also the on time performance of any other airline except AS does not compete with DL.