Ok, here's about a half-dozen reasons why Iguacu is better than Niagara...

1) You're in Brazil. 'Nuff said.
2) A few more steps and you're in Argentina or Paraguay. Way cooler than Canada.
3) It's about ten times bigger.
4) And you can go in it. Literally. A boatride in Iguacu will take you right underneath one of the less powerful downpours, letting you soak in the experience to the fullest.
5) The churrascaria at the Hotel Das Cataratas-- I dare you to find any restaurant in upstate NY as good as this upscale hotel, located in the National Park. The picana is amazing and you can eat as much of it as you like.
6) The Hotel Das Cataratas Bar-- I sampled various aged cachacas ranging from 18 years to 50 years old and they were the perfect way to cap the day.

It's not even close.