Personally, I'd start flying UA. Nonstop service shaves about 2 hours of flight time off the schedule. That's a lot when you are regularly flying long haul. Also, UA is the only airline that has E+ seating which you can use once you have status. It's really nice to have a few extra inches of legroom.

UA might comp you premier based on AA gold - but why waste your one lifetime comp. You might have greater future need.