Starnet is the award inventory system used by the star alliance. Skyteam used a different system.

United is known to block flights on routes that they fly. For example, Air New Zealand from LAX to LHR is never available to Mileage Plus people. EVER.

This is silly and inconvenient because sometimes the partner flight is more conveneint. The lame excuse they give is that "the partner has not made this flight available on United". Whatever. So if you want flight that leaves at 1pm, you are forced to take the United flight that leaves at 10pm. Lame.

CO does not star block. If the flight is available, the one pass agents can book it. This makes the program more valueable, because you can fly the flights you want (on award ticket). I deposit all my miles to Onepass for this reason, and next year, (depending on what shakes up), probably will make mileage plus zero, but keep CO Platinum Elite.

CO miles are far more valuable. The routing flexibily and the absence of any starnet blocking. US does not block either.