API vs standard routes for internal ajax urls

Hello,

I have a user signup form and I'm validating it using ajax. Username must be unique so I use ajax to check for it.

Ajax needs an URL where to send the post request. This URL is for internal use only. No external application / domains / sites will use it.

I was wondering what's the best approach between using a normal route or an api route according to Laravel best practices.

Thanks, Simon