If the OP's travel pattern is principally TATL in J on a relatively high fare class, and enjoying upgrades to F, then I cannot see how flying another airline in another alliance is going to improve that. On another alliance, there will be no upgrades to F (except as EXP on AA, but they're limited to 4 and don't work on BA). Sure the OP will get status on another airline but will not be getting upgrades and will "suffer" the same degradations as he/she would flying UA J.