When is Services not needed?

I have a requirement to have a client hit a URL with some json data, and have the site belonging to the URL accept the data and deposit it in a mongo db for later processing to update Drupal mysql data. Likewise, to have the client be able to retrieve data deposited in the mongo db.

This is D7. Looking at Services 3, I'm unclear whether I would handle this as REST (there are no standard Drupal entities being affected), RPC (though I don't see a JSON-RPC server), some other server, or not to use Services at all and just register the two url's from a module and handle it that way.