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 YYZ and YUL transborder lounges are both comparable in that they are mediocre at best.
YYZ is a bit better going DOM-US because I find the Global Entry process better in YYZ - the YYZ US immigration agents take your form and wave you through, the YUL ones tend to like to play 20 questions with you even though you have Global Entry / Nexus. YUL is better going US-DOM because the OSS process is better with Nexus machines and no bus; you can be at your domestic gate in 10 minutes max with no waiting around.
You've made the same choice I would have.