The short answer: the Canadian side has the better view, because you are looking back at both the American Falls and Horseshoe Falls. The American side has the closer experience — you can stand right at the brink and feel the water leave the cliff.
For a one-day visit from Toronto, the Canadian side wins for almost everyone: better hotels, more restaurants, the illuminations, fireworks, Clifton Hill, and easy access to Niagara-on-the-Lake and wine country. The American side is worth a half-day if you have more time and your passport is in order.
A detail many guests miss: Hornblower (Canada) and Maid of the Mist (US) both run boats into the basin of Horseshoe Falls. The experience is essentially identical — pick the side that matches the rest of your itinerary, not the boat.