We went in late November. This is late spring in SA. It was in the 70s in Santiago and Valparisio, and in the high 60s in Puerto Montt. It was very cold once we got into the Straits of Magellen (lots of rain too), and with the wind in Puento Areas it felt like 20 (it was probably in the mid-40s). Ushuaia was also very cold, but clear and beautiful. It was unseasonable warm in the Falklands...right around the low 60s, and sunny and calm. This is unusual any time of year.
We took tropical and winter clothing. After leaving the Falklands we packed all our winter clothing away. We took down jackets and also took some of those plastic bags that you can compress to get out the air before packing. They worked very well. It was in the 80s in Buenos Aires and in the 90s in Rio with high humidity. Later in the year (ie, Feb.) it is even hotter.
You really need a full range of clothing, and of course plan to layer. Don't forget both gloves and a stocking hat so you are comfortable out on deck to take photos, etc. in the glacier areas and at Cape Horn.
Last december from the fjords through to the Falklands it averaged 6c-10c air temp but with wind chill across deck it felt much colder. Force 7 several days & force 8/9 one night. On the other hand back in the 1950's I was in Montevedeo when it reached 110F in mid january