I've never transferred through YUL (only O/D travel a long time ago). Are the YYZ and YUL US/Transborder MLLs comparable? Also, is the US immigration process comparable between the two airports?
The YUL Vichy CATSA is (or was a few weeks ago) in a seemingly temporary closet, but it goes quickly. US CBP is the same. I've never been to either at "problem" times, though.
Lounges? YUL TB is smaller, but I've not been to it packed. Same selection. Or at least, not wildly different in the slight inconsistencies you have in any DOM or TB lounge.
I transit both frequently. Security/customs/lounge play 0 factor in which I choose.