I have heard mixed results with Air Canada's service. Some people think it is really good. Others have said the airline's service is poor. I am sure both airlines' service for international flights should be better than their domestic service.
AA flies to more destinations in Latin and South America so their presence in the area is better. Therefore they may meets the needs of the passengers flying down to those areas better.
But a question for you to think about is this. Will you be flying again and with what airline? Air Canada is in the Star Alliance. Therefore if you fly any one of the Star Alliance airlines, then you can earn mileage for a free ticket. If you will be traveling American Airlines or any of their OneWorld partners in the future, then fly American on this trip. It sounds as though you will be able to rack up some mileage on this trip so whichever airline you go with, ask to become a frequent flyer member with the airline or partner airline you think you will fly the most with in the future.
If you will not be flying anytime after this trip, then you can check out seatguru.com to check the seat pitch and seat width for each of the airlines. Otherwise, you should be fine going with either carrier.
You should defiently go with Air Canada. The staff are nicer and I heard that they have alot of TVs on the plane to make your flight more enjoyable. Also, American Airlines in the midst of bankruptcy, so, I wouldn't trust them if I were you. There are always going to be mean people on every flight you take, but I have always encountered great people on Air Canada, and I would never fly with any other company domestically.
Air Canada all the way, chances are you will get a plane with the XM effect (Extreme Makeover: Seat back entertainment) they also typically serve food on non USA international flights, & with the XM effect you will get a new interior design, its best to look at which plane your taking down to see if it has all these perks
Both airlines are established full service carriers, so you should be fine with either. Look at the routing, time of flight, seating chart.
AA is just about out of service.
Dont fly them if even if you could get a flight.