Generally the larger and newer end of the expedition fleet, because size, modern stabilisers and hull design all work in your favour. A very small ship moves more in the same sea, which is one of the arguments against fixating on sub-100 vessels. That said, the honest answer is that your cabin location changes how a crossing feels more than the choice of ship does, and no vessel makes a bad Drake pleasant.
What to optimise, in order:
1. Medicate before you feel unwell. Bigger effect than every hardware decision combined. 2. Cabin low and mid-ship. Avoid the bow, where the anchors and thrusters are and the motion is worst. 3. Ship size in the 130 to 180 range. Steadier than a tiny vessel, and it brings the other advantages of that class. 4. Stabilisers and hull. Real, and the smallest of the four levers.
Bow cabins fill last, so booking early is partly about not ending up in one.
If you're choosing a ship primarily on crossing comfort, that's a signal worth taking seriously in a different direction: consider flying across instead, and spend the ship decision on what your days in Antarctica look like.