I am a little confused as to which I am supposed to use.
The API is an trying to hit accepts regular text post parameters although its response is a JSON String.
Which should I use ?
You Can use
JSON Object Request for this.
StringRequest is different in their parent class and their response. You could find it out if you dig into volley's source code.
JsonObjectRequest extends JsonRequest<JSONObject>
StringRequest extends Request<String>
So, if the response is a JSON String, then you could just use
JsonObjectRequest for convenience, since Volley has wrapped the response to be a