TL;DR:
Workers Sites is basically an elaborate Worker template (which you can modify) and allows you to deploy your sites using the Wrangler CLI — it'll take care of uploading your assets and all, but you're responsible for setting up CI, etc for a continuous deployment experience.
Cloudflare Pages, by contract, integrates directly with Github, which allows us to offer a lot more features out of the box, including running your build, preview URLs, etc.
TL;DR: Workers Sites is basically an elaborate Worker template (which you can modify) and allows you to deploy your sites using the Wrangler CLI — it'll take care of uploading your assets and all, but you're responsible for setting up CI, etc for a continuous deployment experience.
Cloudflare Pages, by contract, integrates directly with Github, which allows us to offer a lot more features out of the box, including running your build, preview URLs, etc.
Hope that makes sense.