There's no need to clear Canadian customs in YYZ on both outbound and inbound. Inbound, clear US customs.

Not all awards on Aeroplan cost less miles than United Mileage Plus. Econo award from US-Asia cost 75k miles on AC but 65k miles on UA. Premium cabins you need less points on Aeroplan. Also do note that AC charges fuel surcharges on AC awards, but no fuel surcharges on *A awards. AC awards do allow more creative routings.

If you fly UA, you get Econo plus if you're elite on UA. However AC Y on the 777 are quite nice.