Have an actual conversation before you commit, and pay attention to whether they ask what you want before they start recommending things. An agent who leads with a specific ship before understanding your priorities is selling inventory rather than advising you. Check their reputation in the polar communities too, including Reddit and the forums, because this is a small industry and reputations are visible.
Worth knowing about the shape of the industry: it's more cooperative than you'd expect. Independent polar specialists refer clients to each other when someone's request fits another agency better. If an agent tells you they aren't the right fit for what you want, that's a good sign rather than a brush-off.
The real distinction is between independent specialists and the large general adventure agencies. Both can do the job. Independents tend to give you the same person from first enquiry through to your return, which matters more than people expect on a trip this complicated.