Independently of each other, Nicole and I have both seen Niagara. Together we visited Victoria Falls in Livingstone, Zambia and there's a Chinese restaurant we went to once with a little rock pool and waterfall in the garden. In our opinion, Iguazu left them all drowning and wanting.
The falls themselves are spectacular but it isn't just that. Victoria and Niagara I think are both bigger but it is the scenery of multiple falls in a row or even semi-circle, arrayed just to look beautiful rather than just a waterfall is pretty awesome.
Tips for Iguazu: 1) Do both the Brazilian and Argentinian side - it costs more but is worth it. These photos reflect some from both sides. If you have to choose just one side, then I'd probably say the Brazilian side is better. Just. 2) Do the boat that takes you near the falls .... BUT be sure to use the waterproof bags. Seriously. Our passports were drenched and Nicole's camera almost died.