Montreal is far superior for the city, Niagara for the casinos. What is more important to you? Niagara has to be competitive since there are three casinos close to each other. Casino Montreal is a monopoly run by the province so they have no qualms about fleecing you.
If you go to Montreal, you will have lots of good value hotels to choose from. Other than summer the hotels and restaurants in that city are among the best deals in North America and the nightlife is superb. Niagara can't hold a candle to all that, but the gambling is better.