In regard to companion upgrades, CO is much better in one respect. As a CO Gold elite, when a companion and I fly NW (with me already confirmed in F), I can only upgrade my companion at the gate for each segment. For example, if I fly LAX to MSP to JFK, I can get the companion upgrade LAX to MSP, and then get the companion upgrade to JFK in MSP, not at LAX -- they won't issue the companion upgrade the whole way through.

With CO, past experiences has allowed me to get a companion upgrade the whole way through, or at least get my companion added on a standby list for the 2nd segment if I couldn't get it to begin with. NW will only start a standby list or issue companions at the start of each segment.