The Canadian side offers you a view of the falls that you simply cannot get from the states. You can get much closer to the Horseshoe Falls from the Canadian side, which are the largest, prettiest and the most powerful of the three waterfalls that make up Niagara Falls.
At night, the falls are illuminated in a rainbow of colors, which can truly only be experienced best from the Canadian side. The Maid of the Mist boat tour also operates on the Canadian side so you can experience it from either country. Full details on the Maid of the Mist tour are located in the American Falls review.
Besides the spectacular views, where Canada really wins this competition is in the atmosphere. Granted, it is completely touristy, and I realize some people are turned off by that. There is a Hard Rock Cafe, a Starbucks on every corner, and a bunch of chain restaurants and hotels. But with that said, you can still find some great local attractions and ultimately the nightlife and daytime atmosphere, offer more variety.