As already mentioned, your premium class ticket will get you into the MLL (though will not grant access to priority security - you need to have certain Amex cards or be AC elite or premium cabin for that).
MLL is the better lounge. And per the last time I checked, Premium plaza lounge is still in the temp. Location, in a basically walled off area around gate 54. So no question that MLL beats that.
And just a point of clarification, on UA out of YYZ you’ll be flying ‘United Business’. Even through it’s the same as domestic F service, to Canada, it’s called business.