Hi,
You have 2 options. I am not sure of availability on the YUL-LGA route for AA, so please check that. If you find there is space, then the best bet is going to be using your Chase Sapphire Ultimate Rewards transfered to British Airways Avios. Using Avios, you can redeem 7500 Avios for 1 one-way in economy YUL-LGA or YUL-JFK (any flight under 600mi). The flights you can redeem these for are on AA award travel. AA award travel availability has become abismal the last year or so. Please check that.
Thus a roundtrip wil run you 15K Avios + $60 in taxes.
Using Aeroplan, a flight will run you 15K Aeroplan + $160 in taxes.
Having both options for redemption is good.
In general, though, cash fares are pretty cheap. So you might still be better off doing a revenue ticket from Chase Ultimate Rewards where you get 1.5 cents towards travel. The 15K you transfer to BA Avios could buy $225 of travel. Depending on the flights you are booking, you might be better off with the revenue tickets.
Good luck.