As one who has performed the "Toronto Shuffle" a number of times, I'd say that there still is a considerable difference in the overall products. BA wins on consistence of service excellence (AA service quality varies widely), lounge experiences (no contest), and better catering (wines, especially champagne, are much better on BA). The physical AA seat is a little better in that it swivels (VERY convenient); I consider the seats equal when in the bed configuration.
One caveat to keep in mind is that you must clear Canadian immigration in YYZ, manually transfer your checked bags, then re-clear security for your ongoing flights. It isn't an especially onerous process, however, sufficient time needs to be allowed - too many people encounter problems when trying to schedule the ongoing segment too close (time-wise) to the incoming arrival time.