Please check the following for the difference between Salesforce Web To Lead/Case and REST API for creating Leads/Cases via Web Forms.
Limit Per Day:
- Web To Lead supports only up to 500 Leads in a 24–hour period.
- Web To Case supports only up to 5000 Cases in a 24–hour period.
- Salesforce API supports 100,000 + (number of licenses x calls per license type) + purchased API Call Add-Ons.
Attachments:
- Attachments are not supported in Web To Lead/Case.
- We can do additional REST API cals for attachments.
reCAPTCHA:
- reCAPTCHA is easy to setup in Salesforce without coding skills in Web to Lead/Case.
- reCAPTCHA has to be configured in the Site.
Client-side Validations:
- Hard to do Client-side validations for Salesforce Web to Lead/Case.
- JavaScript validations can be done in the Site before calling the Salesforce API.