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:

1. Web To Lead supports only up to 500 Leads in a 24–hour period.
2. Web To Case supports only up to 5000 Cases in a 24–hour period.
3. Salesforce API supports 100,000 + (number of licenses x calls per license type) + purchased API Call Add-Ons.

Attachments:

1. Attachments are not supported in Web To Lead/Case.
2. We can do additional REST API cals for attachments.

reCAPTCHA:

1. reCAPTCHA is easy to setup in Salesforce without coding skills in Web to Lead/Case.
2. reCAPTCHA has to be configured in the Site.

Client-side Validations:

1. Hard to do Client-side validations for Salesforce Web to Lead/Case.
2. JavaScript validations can be done in the Site before calling the Salesforce API.