I normally fly NYC/MTL with an AC ticket and MP credit instead of the UA codeshare because, for whatever reason, the miles post more consistently. I've even had some luck talking AC agents into reserving Aeroplan Elite seat, and enjoyed the occasional operational upgrade.

As far as I can tell AC and UA now (again?) both allow same day stand-by on non-refundable fares for transborder flights and charge C$150/US$100 for confirmed changes. Anyone know if this is up to date, and if there's any other advantage for ticketing on AC vs. UA if I'm probably going to have to change the return flight on a non-refundable ticket? (This is for the very near term so I'm not worried about UA tickets being accepted, but I would like to help out if the codeshare gives them revenue.)