When is Next.js better than Gatsby?
They can both help with server-side rendering, but in 2 different ways.
The end result using Gatsby is a static site generator, without a server. You build the site, and then you deploy the result of the build process statically on Netlify or another static hosting site.
Next.js provides a backend that can server side render a response to request, allowing you to create a dynamic website, which means you will deploy it on a platform that can run Node.js.
Next.js can generate a static site too, but I would not say it’s its main use case.
If my goal was to build a static site, I’d have a hard time choosing and perhaps Gatsby has a better ecosystem of plugins, including many for blogging in particular.
Gatsby is also heavily based on GraphQL, something you might really like or dislike depending on your opinions and needs.