Comparing Submit JSON to Tally is not very straightforward. While both products handle form submissions, the use cases and applications between them are quite different.
Tally is a "form builder" where Submit JSON is a "form backend".
Developers will generally choose Submit JSON, whereas non-technical folks will generally prefer Tally.
Tally could even be the correct choice for your web project. If you do not care how the form looks or behaves, or simply need to gather some information in a pinch, give it a shot!
But if you'd like to maintain control over your UI and UX, or do something fancy like notify yourself on Discord when a new user subscribes to your SaaS, Submit JSON is the more appropriate choice for your project.