Just to add to acraven's comments, don't go to Montreal during the Grand Prix (early June) weekend! The locals I've asked are not fond of the annual madness. Plenty of other festivals all summer, and also during the winter if you're hardy enough.
Do not not not rent a car in Montreal.
I agree with this wholeheartedly, and likewise for Toronto. I fly into both cities off and on, and have taken the inexpensive UP Express from Pearson Int'l to Union Station (Toronto) then walk to my lodging nearby. If you get a PRESTO card, you can load it up with a reasonable amount ($10 CAD minimum to load) and use it on the TTC-Metro and streetcars are excellent. In Montreal, I by a transit card at Trudeau Int'l and ride the 747 Express bus (not the most comfortable, it's basically a city transit bus) into the downtown core. For the $10 (CAD) fare, you have 24 hours from purchase to use the Metro. Very economical and efficient.
I recently visited Montreal and my flight arrived early evening. The 747 absolutely cruised into downtown, past all the construction. It was the quickest trip into downtown I've ever experienced (daytime, summer, not so much).
Hope this helps.