We were on the Westbound SA trip last January and thought it was the most spactacular scenery we have seen. The East side of SA was very hot; there was a fun "crossing the line (equator)" ceremony to participate in (sign up) and Rio was very interesting, but you will want to have tour with a group because of the muggings, etc. As you go down the coast it will cool to about 50 F, and can be breezy. I know you will have fewer ports of call, but to me, the circumnavigation of Cape Horn island and the transit up the Beagle channel was breathtaking. My wife wanted to shop for leather and either Montevideo or Ushaia (sp?) were good stops for this. Because of the Antarctic currents, the West side of the continent is somewhat cooler, and more overcast, but the cities are very rich in history and interesting. If you're traveling on a US passport, a visa for Brazil is the only one you'll need. We also got yellow fever shots. I have some pictures of our trip at:
http://travel.webshots.com/album/548232658wuXvlQ
I would definitely do this trip.
Rick J