The Arctic has land mammals. Antarctica does not. Everything else follows from that.
In the Arctic you can see polar bears, Arctic foxes, reindeer and musk oxen in the right regions, plus walruses, seals, whales and enormous seabird cliffs. In Antarctica the wildlife is penguins, seals, whales and seabirds, and nothing walks on four legs.
The second difference is people. Svalbard, Greenland and Iceland are inhabited, with communities, culture and history you can visit. Antarctica has no permanent Indigenous or civilian population at all.
Counting species is the wrong way in. What decides it is the kind of experience you want.
Antarctica gives you scale and concentration. Very large colonies, whales feeding in bays, and a landscape with no human settlement in it. It is a wilderness in a way almost nowhere else is.
The Arctic trades that for variety and for the search itself. A polar bear is a hunt, not a certainty, and finding one after days of looking is a different kind of satisfaction from walking into a penguin colony. You also get walruses, narwhals in the right places, and towns where people have lived for a very long time.
Plenty of people do both eventually. Neither is universally better, and anyone who tells you otherwise is describing their own preference. If you are choosing between them, tell us which of those two experiences you are actually after and we will be straight with you about which trip delivers it.