If you are worried about Lounge access as an AC vs UA, don't

With both US and AC, with *G you will have access to a nice transborder lounge in YUL.

The lounge in LGA is outside of security and a good 5 min walk to AC's gates. The best thing you can do in LGA is come as late as you can. If you have to do this weekly, I hope you have a lot of patience (LGA is a horrible airport).

I would also recommend a flight pass, the Latitude option is by far the most stress free thing you can do. Latitude Flight passes usually go into M class which is one of the higher economy fare buckets, so it keeps you pretty high on the standby list (if you choose to go early). The Es are excellent planes, especially in J (and very good in Y, too).

If you are able to rack up 100,000 miles and gain SE, you will have access to the SE awards, which pretty much opens up every seat for you (in both Y and J) for points flights. It has to be on AC Metal, but if you are cross the border so often, what is one more flight.

and to echo Zorn, if you have not already applied for Nexus you have lost time!!