If your service works fine with only models and the GET/POST/PUT/DELETE pattern, use pure REST.
I agree that HTTP is originally designed for stateless applications.
But for modern, more complex (!) real-time (web) applications where you will want to use Websockets (which often imply statefulness), why not use both? JSON-RPC over Websockets is very light so you have the following benefits:
As you are only designing the server side API, start with defining REST models and later add JSON-RPC support as needed, keeping the number of RPC calls to a minimum.
(and sorry for parentheses overuse)