1. If you do not have BA status then booking seats in Club is outrageously expensive - so a plus for Air Canada.
2. Air Canada lounges are far superior (in my opinion) and the Signature Lounge at Toronto is very pleasant.
3. Concierge service in AC can be absolutely brilliant. I recently got off a flight from YYG transiting at YYZ to LHR. AC representative as we disembarked in Toronto met us and took us by back stairs to an Audi Quattro then directly to international departures close to the Signature Lounge - Brilliant.
Trouble is that AC is usually more expensive!!