What they said. I am based in Chicago, but work in Montréal, and fly mostly on Air Canada. Nonetheless, it is better for me to credit all the points to UA's programme, as my transoceanic flights tend to be on UA, not AC. I am 1K on United, which gets me lounge access when I am traveling internationally (including to Canada) which is most of my travel, and upgrades on my longer haul flights which tend also not to be on AC. If however I were going to be doing a lot of longer flights on AC, I would probably benefit from being an elite Aeroplan member.