When comparing these two products, the main differences that stand out are cost, plan limits, and overall design philosophy.
A Formspree "Business" plan costs a whopping $102/month with 10,000 total submissions and 20 total forms. Compare that to Submit JSON's $15/month plan with a submission limit of 10,000 and 50 form endpoints. We beat them almost by a factor of seven, yowza!
But we must give credit where credit is due, because Formspree is the granddaddy of form backends. They were simply winging it and everybody else followed suit! If it wasn't for them, Submit JSON would never exist.
Their HTML-first approach is their greatest strength, and also their greatest weakness. In comparison, Submit JSON's API-first approach is more secure, versitile, and aligned with modern web development best practices.
Which is the best form backend for you? We think the answer is obvious, but we'll let you decide.