Either soft or hard-shell works, and the bag type matters less than the weight allowance. A soft duffel compresses better in a compact cabin, a hard case protects fragile equipment. The real constraint is the baggage limit on every flight, especially charter and domestic sectors into Ushuaia or Punta Arenas, which can be well below your international allowance.
Check the exact limits on your own itinerary rather than a packing blog's example. On fly cruises the charter sector is the tightest link in the chain.
Leave space for the way home. If your operator gives you a parka to keep, or you buy anything, it has to fit.
Pack your carry-on for the scenario where the checked bag is delayed: passport and documents, prescription medication, glasses or contacts, chargers, camera bodies and spare batteries, one complete base layer set, underwear, socks and a light midlayer. If the operator supplies the parka and boots, a delayed bag is survivable as long as you have what goes underneath them.