I belong to both BAEC and Aeroplan. In my humble opine, in general, FF miles are only of significant value for obtaining travel in premium cabins.
It is WAAAAAYYYYY

easier to get a premium cabin on BA than on AC.
As added bonus you can have the option upgrade to Club class from a WT+ ticket.
BAEC for TATL travel is an easier option for premium cabin travel.
You can also get more than two CW seats - often up to 6 seats can be available on a given flight.