If you're a solo founder or a creator focused primarily on email marketing for newsletters, go with Kit (ConvertKit). Its tag-based subscriber management and strong visual automation builder cater to creators effectively. You'll have access to creator-friendly landing pages and forms, which are beneficial for growing your audience. However, be mindful that the email design is functional rather than fancy, and costs can increase as your subscriber list grows.
On the other hand, if you're running a media business or an established newsletter operation that requires growth tools, beehiiv is your best bet. Its built-in referral systems and monetization options through ads and subscriptions are ideal for driving revenue, but remember that these features are more useful when you're ready to scale. While it has a polished editor and strong analytics, it may not suffice for detailed lifecycle emails or automation tools suited to individual creators.