The positives are both are pretty similar but the usecase is different.

Gatsby probably isn't the right choice if you have a lot of pages (e.g. ecommerce site) as it has to render each page down to HTML during the build - so everytime you make a change, you have to rebuild all those files which could take a substantial amount of time. But if you were just building a few landing pages or a blog that required good SEO then Gatsby would be fine.

Next.js renders serverside on the fly and has good SEO.

This might be useful for you to read: https://coffeencoding.com/cra-vs-next-js-vs-gatsby/

Next.js is server-rendered which means it includes a light weight node.js server that pre-renders your pages, you can export everything as an static website, but it's not really the main selling point.

I like to use next.js for small brochure-like websites. I think it's simpler for simpler things. But for a more full-featured content site I would chose gatsby.

Would be interested in this as well. I currently use Gatsby for a personal site, and the development server is great and for prod I just sync production assets to an s3 bucket and serve from there. It's hard to see how it could get any easier.