The choice between using single-file pages and pages with code separation is primarily one of convenience and personal preference. Working with both models in Microsoft Visual Studio is similar; both models have approximately equal support in the editor. Both models have equivalent performance when the page runs. The page with code separation makes it more practical to let a Web designer work on the layout of a page while a programmer creates the code for the page, because the two pages can be edited separately. https://msdn.microsoft.com/en-us/library/kxbhbb82.aspx
Choosing a Page Model
The single-file and code-behind page models are functionally the same. At run time, the models execute the same way, and there is no performance difference between them. Choosing a page model therefore depends on other factors, such as how you want to organize the code in your application, whether it is important to separate page design from coding, and so on. https://msdn.microsoft.com/en-us/library/015103yb(v=vs.100).aspx
