With the Brazil side, we were in the Falls, while on the Argentina side, we were on top and below the falls. Very different perspective, in Brazil side, you will get very wet. Personally, I enjoyed the Brazil side more. However, there is only 1 trail on the Brazil side, so it'll only take about 1 hour to finish the trail. From timing side, I think adding the powerboat ride on the Brazil instead of the Argentina side would make sense to make the crossing into Brazil 'worth' the effort.
The Argentina side will take more walking with the three trails, and waiting for the train.
Either way, I think if budget permits, staying at either the Gran Melia on the Argentina or the Belmond on the Brazil side is a game changer. Makes it so convenient to step out of the hotel and into the Falls park.