Originally Posted by
bosman
WT+ is a separate fare class, so you cannot get into those seats with a regular coach fare (whereas MCE is a special seat, but same fare class as regular coach on AA). Plat will allow you to preselect a seat in coach on BA whereas, without that, you would either have to pay extra to preselect a seat or wait until T-24 to select your seat.
Note that WT+ on the AA codeshare often costs considerably more than on the BA code, and you will earn more miles with the BA flight number (they give a 10% bonus for WT+). Also, the AA codeshare WT+ will probably not be upgradable with AA miles, but the cheapest BA WT+ is upgradable with Avios (should you have any).