An amazing place.... we just wanted to stay there and watch the waterfalls forever. The argentinian side is muh longer and brings you close to the waterfalls. Also in the argentinian side one is just meters away from Garganta del Diablo. On the other hand the brazilian side offers more open perspectives and thus incredible panoramic views. Long story short one should visit both sides.
In the argentinian side we have spent a full day. During the morning the sky was foggy (see the first pictures) but after 1 pm it cleared away so we have repetead the Upper and the Lower trails for a second photo round. We crossed to San Martin island, took the boat for a closer waterfall look (and of course we got completely wet) and at the end we have returned from the Graganta del Diablo by a man powered boat.
The path in the Brazilian side is much shorter so we did it both ways (go and back) in around 2 hours. The panoramic views from this side are incredible. The drawback is the distance to the waterfall that is much bigger. Also one cannot visit the falls one by one.