Personally, I would go for the Marriot. The Sheraton is great, if you are on the Club level. The club lounge is one of the greatest of any Sheraton's I have ever stayed in--truly fantastic. But you're right--the price is too much.

The Chateau Champlain is a great landmark building in Montréal. It is connected to the "Underground City", which can be just as important in Montréal's hot, muggy summer, as it is in the freezing winter! The rooms are a good size, and all have those bayed, floor to ceiling, semi-circular windows.

But if the price differential is too much, you could certainly go for the Four Points.