There are parts of Quebec City outside the wall that look exactly like Montreal but they are mostly not shown here (eg, Limoilou, however, a bit of Grand Allée resembling Montreal is shown). On the whole, Old Montreal and Old Quebec don't really resemble each other that much. It is important to note that Quebec City fell under British control and a lot of the Colonial era architecture in Quebec City was built by the English but took on local flavours as it was specially adapted to the cold, snowy winters of Quebec City. Quebec City boasts many architectural styles due to its long history.
Old Quebec:
Old Montreal:
Quebec City is characterized by its more pedestrian oriented streets and public squares. Montreal has many more larger and newer (early 20C) buildings mixed in. Generally I think Quebec is more coherent than Montreal. Montreal feels more like a large old American city.
Montreal may be the big French Canadian metropolis, but Quebec City has always been the capital, which means there are many grand public buildings, parks and an overall design that reflects its status as the "national capital" (for those who don't know, Quebec's government conducts itself as a national government).
Linguistically, Quebec is very French. Virtually everyone in Quebec City speaks French at home and when socializing. English is only spoken because of tourism. Historically, Quebec City had a sizable English population, but they have become Francophones over time. Government business is conducted in French (not historically the case). By contrast, Montreal continues to have a sizeable English only population and has conducted much of is business and commerce in English. On the whole, many languages are spoken in Montreal, which is much more ethnically diverse than Quebec City.
Other things that contribute to Quebec City's uniqueness are its topography, its vibrancy, the scale of its historic areas, and its level of historic preservation. Notably, Quebec City is the best preserved walled city in North America.