The answer depends on what's more important to you. Do want to live in Montreal or make more money living in the US ? You cannot have it both unfortunately. I wouldn't worry about future issues though if changing countries. You can always negotiate a higher salary while moving back to US if you are worth it.

So for short term, if you want to be in Montreal, go for it. In the future if/when you want to move back to US, just look for a job and ask them for a salary that you think is a good match then. Don't sweat it right now.