The verdict
For most small B2B teams, I'd pick Mailchimp. Nobody needs training, the template library keeps emails on brand without a designer, and most small stacks connect to it without custom work.
If your business is the audience itself, such as a paid newsletter, a course or a creator brand, pick Kit. Its commerce tools and tag model are built for that.
Neither is a real sales tool. Mailchimp contact profiles are basic, and Kit has no deal pipelines. If you need branching nurture or CRM sync, look at ActiveCampaign or HubSpot rather than stretching either.