Assuming switching programs will not change your flying patterns, I would not recommend the move. UA Premier Executives (=US GP) do earn a 100% bonus on all UA and US flights. But if you spend most of your time on US planes but accrue to UA's program, you won't qualify for most special promotions of either airline.

Furthermore, as a UA elite you won't qualify for automatic upgrades on your US flights, and by the same token the time you spend on your US flights won't earn you any UA upgrade currency for your UA flights.

You could try to take UA flights where you currently take US flights, but UA has very sparse, mostly United Express service on the East Coast-- featuring the hated Dulles airport (and its especially despised G Concourse).