I totally disagree with Intr3pid on small ship v large ship for this trip. We did the Ponant Antarctica cruise a few years ago. It was by far the best trip I've ever done. The level of service is amazing.
We left from Ushuaia and the trip south was so smooth the captain was calling it Lake Drake. We made such good time that we got to do an extra excursion. We did 12 excursions in all. Some were on land (did you know the Andes actually go undersea then pop back up on Antarctica...we hiked on them.) Some excursions were in rubber boats to see seals or icebergs. We had minke whales swimming under and around our boats and humpbacks breaching 30-50 feet from us. I got amazing photos. One excursion was onto an ice floe. The ship is small enough to sail into a caldera ...a collapsed volcano where just the ring is above water except for a narrow opening. On shore, if you dig into the sand, it is hot.....steams...volcano still down there. We went to two (seasonal) research stations and saw about a million penguins (250,000 pair at one location).
Every night we had naturalists and researchers talk and show slides of what we had seen and what we might see the next day.
On our flight back from Ushuaia to Buenos Aires we spoke with some folks behind us who had been on a large ship. They were blown away by what we got to do and see.
Yes, big ship will be cheaper but this is a once in a lifetime trip....buy the best you can afford.
National Geographic (and likely others) also do small ship cruises there that might be cheaper.
One last point...on the way back north, we paid for the smooth sailing down. We hit a storm that intensified overnight. Twice our entire mattress, with us on it) ended up in the floor. One dining room didn't open for breakfast the next morning. But, I never felt in danger.