When comparing these two products, the main differences that stand out are cost, plan limits, and overall design philosophy.
FormKeep's starter plan starts at $5/month with 1,000 total submissions and 1 single form. Compare that to Submit JSON's $5/month plan that sports a submission limit of 2,500 and 10 form endpoints. FormKeep has the audacity to charge per additional form, starting at $5 per form per month!
Regardless, it's another Formspree clone. Can't you tell by the name? FormKeep's 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.