Lounge access is meaningless if you don't fly much. Air Canada at least provides $25 MLL passes on certain fares, which would be more affordable than the $50 Admiral's Club passes.

You'll only gain full value out of AA if you tend to fly transborder or international, as a YYZ-based customer. Furthermore, because of the American nature of AA, you'll have to make connecting flights a lot more than if you were to fly AC, which hubs in YYZ.

Unless you have a pressing reason to fly AA or BA (and not discount economy either), I'd stick with Air Canada. At least you'll get reasonable accumulation offers from aeroplan partners.

Of course, if you can't reasonably get 25k miles in seven years, take AA any day. That expiry clause on aeroplan is pretty brutal in that regard.