Using WebAPI or MVC to return JSON in ASP.NET

Web API Controllers can be created and hosted in any ASP.NET Application, not just MVC applications. Thus, an obvious reason to create a Web API is if you do not have an MVC front-end (e.g. classic, RESTful web-services hosted by your company/organization.)

MVC Controllers typically rely on the MVC Framework, if you look at default templates and most of the work done by the community and your peers you will notice that almost all MVC Controllers are implemented with the View in mind.

Personally, I use MVC Controllers when I intend to respond with a View(), and I'll use a Web API for anything that isn't dependent on a particular view.

There are caveats, of course, but generally speaking if you don't require the Model Binding behavior of MVC, your service is data-centric, and operations are Data-centric (e.g. CRUD operations) then you likely want a 'Web API Controller' instead of a 'Model-View Controller'. Conversely, if your operations are View-centric (e.g. delivering a user admin page to the user), or you need MVC's Model Binding to generate 'ajax partials' (very unlikely), then you will want an MVC Controller instead.

Personally, I use Web API controllers for driving JSON-based RESTful clients, I use MVC controllers for handling basic browser routing and delivery of the SPA.

WebAPI is for making an API. If you want someone to be able to consume your API in XML, JSON, etc. You can make a web api.

In your case you only need to talk to client in JSON.

Even though your website is mostly client script driven you would still be using ASP.NET MVC Controller right? And since you may have already logically divided your controllers based on entities then it make sense to add those json serving methods in it as opposed to making another class specifically for web api.

So for your particular situation (if i understand correctly), I would stick with Controllers.