Doug - A few years ago, on a land trip in Chile & Argentina, we spent several days at Iguaza Falls and would not have missed it for the world. It is one of the most memorable places we have ever visited. We stayed on the Argentinian side (the International Hotel, I think, which looks like a typewriter, but has great views of the falls from all of its rooms). We saw the falls from the Argentinian side, the Brazilian side, from the air (helicopter), and from the water (Zodiacs which went right into the falls!! - great for memories, bad for cameras...). If you want a sense of the grandeur, get the DVD of "The Mission" - the scenes of the soldiers dragging their equipment up the cliffs and of the bad guys going over the falls were all filmed at Iguazu (guided by the guy who runs the Zodiac trips).
We cannot yet opine on MP, since we had to cancel our trip there this year due to illness; but we hope to make it next year. We will be eagerly watching for reviews of the options. In the research we did, we determined that the Hiram Bingham train was not remotely worth the extra $$, which could be better spent on staying at the expensive, but wonderfully located, hotel right at the ruins, the Sanctuary Lodge.
Cheers, Fred