Big question -- no simple answer. That's why Flyer Talk exists. Look through the various threads. There may be a super bonus miles promotion on Air Canada one month that is just too good to pass up. Cost may be another reason--United has great e-fares and now a great sale with prices to Europe that are half of Air Canada's. Perks and upgrades are another possibility. As an Aeroplan Super Elite member, I can get confirmable upgrades on deep discount flights on Air Canada internationally and that makes a huge difference. So keep reading and looking, there is lots to learn (I am still learning!)