CO's website has some nice features, but so does UA's (the ability to view inventory buckets is a great feature). This is more about the backend systems.

UA's airport and reservations infrastructure is, I think, superior to CO's - I've been rebooked and rerouted in the space of 30 seconds on UA which it sounds like is far more laborious on CO. In addition, waitlisting for upgrades seems marginally better on UA (I don't quite understand why on a YBM-up on CO, the passenger has to keep checking for available space once it's booked). Finally, UA seems to be able to handle upsells at any point in the check-in process (OLCI, EasyCheckin, checkin agent, RCC agent, gate agent, customer service agent) that CO does not yet have?