I think you'll be OK either way. If the YYZ-GRU flight is late, I imagine they hold the plane in EZE as it's half the passengers. If the YUL flight is late, then everyone has to wait because that seems to be the plane that continues on. I believe my bag was checked all the way through, don't recall having to pick it up and re-check in GRU.

As an FYI, probably been said on here before, you can't bring liquids through GRU on your way to EZE (e.g. you can't buy duty free in Toronto and bring it with you all the way). They'll confiscate it in GRU as you have to go through a special security check.