When comparing these two products, the main differences that stand out are cost, plan limits, and features.
Netlify Forms starts at $19/month with 1,000 total submissions. After the 1,000 submission threshold is crossed, it costs an additional $9 per 500 submisisons.
Compare that to Submit JSON's $5/month plan that sports a submission limit of 2,500, and their $15/month plan with 10,000 submissions.
Netlify's strategy mirrors that of Apple - they first create an ecosystem, then monopolize it by selling you solutions that only work in said ecosystem. From this position they can charge out the wazoo, because they've already caught you in their trap.
In comparison, Submit JSON was designed to be affordable, versatile, and generous.
Which is the best form backend for you? We think the answer is obvious, but we'll let you decide.