TL;DR
ConvertKit if you are a creator: newsletter writer, course seller, podcaster, indie business. Mailchimp if you run a small business / ecommerce site sending broadcast campaigns to a customer list.
Where ConvertKit wins
Sequences, automations, subscriber tagging, and landing pages all built around the creator workflow. The interface assumes you are thinking in terms of "audience" not "customers".
Where Mailchimp wins
Drag-and-drop email design, ecommerce integrations, Shopify/WooCommerce sync. If your primary use is "send a newsletter to a customer list", Mailchimp is still credible.
Pricing reality
Both scale with list size and get expensive fast above 5,000 subscribers. Mailchimp tends to nickel-and-dime with feature paywalls; ConvertKit is more all-inclusive at each tier.
What about Substack?
Substack is a different category — it hosts and monetises newsletters but you do not own the list relationship. Pick Substack for content distribution + revenue split; pick ConvertKit if you want to own the email channel.