YUL is much more preferable. As for bus versus train, I'd personally go for Via Rail over a bus like Orleans Express unless you're looking to stay much closer to the bus station on the east side of downtown Montreal. The train is nicer than the bus. Prices are very comparable for either and they leave from the same point in Quebec City. If you're not looking to stay in Montreal, then the bus drops you directly at YUL with a transfer at the bus station. If you take the train, the Dorval station next to YUL is served by Quebec City - Ottawa with no connection and you just take the shuttle service over.