Go with United and the Star Alliance-- 100% mileage credit on trans-Atlantic trips, easy upgrades to business on United, and Economy Plus available to people with status (regardless of fare) if you don't upgrade.
Plus if you move back to the US, UA is a better choice; BA constantly screws US residents over with only a fraction of miles flown credited as frequent flier miles (perhaps a payback for 1776?). Why put up with that?