One of the most dramatic waterfall systems on the planet — split across two countries, and most people only plan for one.
The honest answer: both, if you have the time. The Brazilian side gives you the panoramic view — you see the whole falls system from across the canyon in a couple of hours. The Argentine side puts you inside the falls, on catwalks right over the water, and takes closer to a full day. If you only have one day, most first-timers are happier with the Argentine side; if you have two, do both.
Check your passport's current visa requirement for Argentina separately from Brazil's — they are not the same rule, and this trips people up constantly. Bring your physical passport for the border crossing even if you entered Brazil digitally.
Cuts your travel time to the falls to under 15 minutes each morning.
More restaurant options, still a short drive to both sides of the falls.
Yes, if you plan it right: one day per side of the falls, and skip trying to add a third activity — people who try to cram in the bird park and the dam and both falls in two days end up exhausted and remembering none of it clearly. Less, done well, beats more, done rushed.
— Aaron
One day for each side of the falls, one slower day for the birds and the sunset. Two pages, made for your phone.