I would take the US comp status for the benefits of check-in and security lines, pre-boarding and priority status. However, I would make sure all miles are credited to the UAL M+ account as to build status.

Use the US benefits while accureing UAL status. I am not sure this will work but it is worth investigation. Just remember, you will loose your one-shot status upgrade with US but if UA status is what you are looking for, then it shouldn't matter.