If you are flying UA roundtrips more often than RTWs then I'd stick with UA. The reason is that upgrades on other airlines using the *A mileage upgrade program are very expensive. You will get far better bang for your mileage buck by using the miles upgrading on the airline of your chosen FFP, or on awards (especially business/first class).

As an illustration, in economy class a simple RTW like you have booked will earn between 20 and 50k miles. Barely enough to upgrade one flight on another airline. Those same miles can be used more effectively as an award.