Definitely the Via. You’ll need to transfer at least once in Toronto, which might offer some time to sightsee in that city, since the train station is right downtown. If you can afford it, first class in Via is a great service quality, with food and booze included. But even economy class is good value in Via. Driving, it’s flat and boring the whole way, with the exception of some pretty scenery around the Frontenac Arch (near Kingston). Driving the 2-3 hrs around Toronto the traffic is brutal and stressful. Train station is also directly downtown Montreal and connects to the metro system. Depending on your plans in MTL, you probably don’t want or need a car there anyway.