Although UA publishes fares LGA-YUL, you'll end up on AC codeshares, so your only choice is between AC and AA operated flights.

With status, you get lounge access on both carriers in LGA, but only AC in YUL. Pricing is similar.

AA = 4x daily ER135s, all economy, better award redemption rates
AC = 7x daily mostly ER175s, roomier aircraft, possibility of upgrades