Substack has great recognizable branding which increases the perceived trustworthiness of the newsletter + their recommendations algo is awesome
But beehive seems more flexible in terms of personalization. Also, if you're planning to monetize your newsletter, beehive 100%, their fees are significantly lower afaik
Both Substack and Beehiiv are excellent choices for newsletters, but Substack is more established and offers a larger community, while Beehiiv provides better customization and growth tools. Ultimately, it depends on your specific needs and goals.