you need 2 days at Iguassu, one on each side without a shadow of doubt.
I agree that Iguacu should be seen from both the Brazilian side and the Argentinian side; both allow amazing but very different views of the falls. I'd actually leave 3 days for Iguacu if possible to give yourself the best shot at good weather (it's best when it's sunny as you can see rainbows, and the butterflies come out) and in case parts of the falls are closed due to the river level being too high. I was there about 5 weeks ago and on my first day at the Argentinian side Garganta del Diablo (probably the most dramatic part of the falls) was closed due to the high river level, so I went back 2 days later just to see that part; it was well worth it. If you visit the same side of the falls on 2 consecutive days you get 50% off the cost of admission on your second day.
I didnt travel from Iguassu to BA but I know this is another long trip
Iguacu to BA takes 18 hours by bus but as Argentinian buses are so good it's not really a problem. I went with a company called Rio Uruguay and they were great.
[ Edit: Edited on 01-Dec-2009, at 18:07 by bex76 ]