Uh, the irony! Indeed, if you want to fly on United, then Aeroplan miles are where it's at. On the other hand, if you want to fly on Air Canada, then fees and taxes will kill you at Aeroplan, but are negligible through Mileage Plus. The difference was more stark prior to the new partner chart at United, but it is usually still better to use each airline's miles on the other airline's flights.