Most people do Antarctica first, and there's a logic to it. Antarctica is a single destination with a clear pull: the seventh continent, the last frontier. The Arctic is a region rather than a destination, covering Svalbard, Greenland, Iceland and the Northwest Passage, with very different trips inside it and something Antarctica doesn't have at all, which is people. Cultural experiences and Indigenous communities are part of Arctic travel.
The pattern we see is consistent. Most Antarctic passengers haven't done the Arctic. Most Arctic passengers have already done Antarctica. The polar bug takes people north after it's taken them south.
What draws people to the Arctic is polar bears first, then the northern lights, birds, whales, hiking and culture. Belugas and walrus matter a lot to wildlife travellers, and there are enough endemic species to make it a serious birding region.
One practical difference: Svalbard has tighter restrictions than Antarctica, set at government level, because it has a permanent population and geopolitics attached.
Alaska is Arctic too, but it's a separate industry with US-licensed captains and different operators, and it isn't what we mean when we talk about Arctic expedition cruising.