The question then depends on where you're based (YYZ has direct flights on AC to many destinations avoiding a connection in the UA hubs) and how many miles you earn. I fly more on UA than AC, but I credit to Aeroplan because I can't do anything with the miles if I credited to MP - I don't fly enough (though when I do usually it's United J). I rely on other spend to get me enough miles to do anything with it. I doubt you'll have that problem if you're 1K. If you can use what you accrue and you either live in a Canadian airport that requires transit thru UA hubs anyway (like YYC), or you don't care about doing so, sticking with UA works for you.