Pick Next.js for
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The React ecosystem
Every component library, every hiring pool, every Stack Overflow answer. If your interface is React and your team is React, that gravity is real and it is worth something.
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Server components and streaming
The RSC model has the deepest production track record and tooling for partial rendering and client bundle size. STX has server components, streaming, and islands too, but React's version has far more teams and years behind it.
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Vercel
Preview deploys per pull request, edge rendering, and analytics with no infrastructure work at all. Stacks deploys to infrastructure you own, a Hetzner box, AWS, or any Linux server over SSH, which is more control and more responsibility.
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Asset pipelines
next/font self-hosts and subsets fonts with no layout shift, and next/image is heavily battle-tested. STX has an Image component with srcset and WebP/AVIF output, but fonts are still a manual link and @font-face.