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This is a matter of preference. That's all! There is no rule that prevent you to go with webforms. Comparing to webforms, MVC is "light" weight in the sense, there is no complex "page" life cycle etc. Apart from that, MVC provide more control to render the HTML. These are the some reasons, that I find personally if I go with MVC and SPA. Hope this helps!
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Have a look of the link this will help you
http://www.asp.net/web-api/overview/getting-started-with-aspnet-web-api/build-a-single-page-application-spa-with-aspnet-web-api-and-angularjs