Location matters more than grade. Lower decks and mid-ship feel noticeably less motion than high or forward cabins, which is the thing most people wish they'd known. Avoid the bow if you can: it's where the anchors and thrusters live and where the motion is worst. Bow cabins fill last, so booking early is largely about not ending up in one.
The counterintuitive part is that paying more can make the motion worse. Premium cabins are often higher up, and higher up moves more.
Beyond location, cabin choice is mostly about how much time you expect to spend in it, which on an expedition voyage is not much. You're ashore, in a Zodiac, at a lecture, or on deck for most of the daylight hours.