I agree that the drink selection at the MLL is better but if you enjoy quiet then head to the Priority Pass lounge since it's like a hidden gem that few know about.

You can usually stay in the Priority Pass lounge a little longer since it's on the same level as the departures gate.

My last visit to the Plaza Premium (YYZ Transborder) was in August 2013 and it was pretty much empty, there was a line to get to the MLL elevators.

You could save those Priority Pass BMO Elite MC visits for the lounges at MIA and use your MLL passes in YYZ. That way you'll get lounge access for both flights.