If you are talking about flying into the Argentina side (Iguazu) and out of the Brazilian side (Iguacu), there are two obvious pros and cons.

Pro: The falls (from 275-350 separate falls) are more interesting when seen from both sides. However, I have only seen them from the Argentina side but was thoroughly impressed and awed. I don't think three days is excessive.

Con: You will need a Brazilian visa to fly out of Iguacu and this is not cheap. However, if you're going to Rio, you'll need it anyway.

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Vic's travels: http://my.flightmemory.com/vogilvie

I did it in reverse, flying from Rio to Iguaco, spending three nights on the Argentine side, and flying from Iguazu to BsAs. Three nights is not too much, you could make a case for another night. I used my UK passport, so the visa wasn't an issue for me.

Thanks so much. And would 2 days in Rio be enough to get an overview of major sites

The big two are Corcovado Mountain (Christ the Redeemer Statue) and Sugarloaf Mountain. With a private guide, you can see those two and more in two days, but maybe not the same day (because of crowds and heat, if going in summer). If taking some sort of organized city tour - two days should be O.K.

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Vic's travels: http://my.flightmemory.com/vogilvie

I didn't do Sugarloaf, but I found enough to do for two days, plus I did a day trip to Petropolis. Click on my name for my TR. I don't think you need a private guide, although you do need to be on high alert.