I'd recommend flying AC and crediting flights to AC and not to UA. You're going to be flying AC anyway most of the time on the route so you'll have a chance to upgrade and to use the RCC in LGA and MLL in YUL without having to buy a RCC membership.

AC over AA for convenience and choice. And if sometimes you can't get on the YULLGA direct, you can always go via YOW or YYZ.