I would suggest the YUL option for a couple of reasons.

There is a direct path from YUL domestic flights directly to US CBP which has it's own security screening that tends to be not as busy as YYZ.

Also, you mentioned that the YUL option is an afternoon connection. The YYZ option involves a morning connection. This is when the US departures area is most congested and a slight delay will put your connection in jeopardy. I fly out of YUL regularly to the US. I always try to get an afternoon flight just to avoid the morning madness.

I strongly suggest that no matter what option you choose you download the US CBP app. You will be allowed into a special US CBP lane and basically be sent to the front of the line.

Also, if there are delays please keep in mind that depending on the reason of the delay AC is legally required to reroute you on any airline they interline with ( DL, AA, UA) especially if the delay is within their control. If the flight is delayed or canceled in Quebec look up available options and insist they rebook you on the spot.