Both work, and the better one is whichever you'll actually wear comfortably. Merino is warm across changing temperatures, resists odour, and rewears well over several days, but it costs more and dries slower. Synthetic is cheaper, lighter, more durable and dries fast, which suits anyone planning to hand wash in the cabin, though some synthetics hold smell. A mixed system is completely reasonable: merino on top, synthetic leggings below.
Two complete sets is enough for most Peninsula voyages. One airs out or dries while you wear the other. Go to three for a longer itinerary, if you perspire heavily, or if the trip adds South Georgia and the Falklands where laundry access is thinner.
Midweight is the safest general choice. Lightweight suits people who run warm or plan active hikes, heavier suits people who get cold or travel at the edges of the season.
Don't replace functional outdoor clothing you already own just because a packing list names a premium brand. Fit, moisture management and the ability to layer matter more than the logo.