If you do not do much intra-Canada travel, staying with AA is fine. Halifax is reasonably served by Westjet for intra-Canada (discount carrier, AIR MILES shopping program). You'll also gain the ability to earn miles on BA for transatlantic travel.

If you were qualifying via EQP for AA status, you'll find flying Premium Economy T-fares on BA to be a huge boon (1.5 EQP, but for less than the cost of H in most cases).

It'll be difficult to not transit the US for Latin American travel if you don't fly Air Canada.