Both Substack and Beehiiv are reliable in terms of deliverability, archive, email list maintenance, and writing experience.
I wouldn't differentiate between platforms based on what's better but on the features you use.
Most people pick Substack because they hear it's free forever. Or Beehiiv because their marketing is sexy, and it looks like the coolest place to host your newsletter.
But here is the truth.
Substack offers basic analytics, web and email publishing, referral program, basic email list management.
If all you want to do is write, publish, and collect emails - Substack is your solution.
Beehiiv offers advanced features. Subscriber management (forms, tagging, automation), monetization options (ad network, boosts), website building (build multiple standalone websites,) and growth options (boosts, referral program, etc.)
If your newsletter/business requires personalization based on data, complex automation, or advanced analytics - Beehiiv is your solution.
Most people use Beehiiv for Substack's features and call it expensive.
But in my opinion, Beehiiv is worth the investment if you actively use at least 60% of its features.