We were very pleased with our S.A. trip. Buenos Aires is amazing and the Tango show/dinner at "Senor Tabgo" is a once in a lifetime experience. We loved the penguin rookeries all be it a pain to get to them. Our ship review is on this site. Just look for the Celebrity Zenith review by me 12-22-02. There may be some information that will help you.
As for ships, I think any of the mainstream carriers do a pretty good job. The ships that are sailing will depend greatly on your sailing dates. There aren't nearly the numbers of ships sailing around S.A. as there are Mexico, the Caribbean, etc.
We only had two choices for our trip. Our choices were the N.C.L. Dream and the Celebrity Zenith. We had been on the Dream in Europe and weren't pleased with the ship, so we opted for the Zenith. We usually sail Celebrity, Princess, and H.A.L. We also would look favorably at an Oceania cruise. I hope this helps your decision.