Two other points to consider:
Being BA Gold/GGL will bring more benefits than just being AA Emerald. BA look better after their own golds than after those of other airline's.
Getting a shiny card on BA is easy if you travel in premium classes. If you travel only in Y it takes forever and it won't be much fun. If your travel pattern is mostly in Y the AA status match option mentioned above may be your best route.
Oh, and welcome to flying BA. When I switched sides (years and years ago) it was a marked step up from UA. Would be interested to hear how you think the two compare these days.