Let me share my thoughts on the topic:
Since you are having in mind an MVC 3 application, the better approach would be to integrate that application into the CMS pages powered by Sitefinity. You can have Sitefinity and the same application in the same project and either run the MVC app in Pure or Classic MVC mode. This will require no rewriting of existing functionality, since Sitefinity supports both modes (link here). For instance, when you create the home page, it will be based on MVC template and it will render data from your application with MVC widgets. So this approach offers you to integrate your MVC 3 application into Sitefinity and use it this way
Another alternative is to utilize jQuery.load() method and use it in your MVC 3 application to get the html generate by Sitefinity. This will be the easiest and the quickest way to allow you to take either the whole page and render it in your MVC 3 application or take just a portion of the page, e.g. the inner HTML of a specific div element. Another good thing is that you will take the most out of our caching and this will be a very fast way for you to take the needed markup and you won’t put burden on the Server for each request. We also have get HTML methods for pages, but they are executed on API level and thus caching will not work.
I hope this helps!
the Telerik team
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