You should use Web server controls for three simple reasons:
Where each of these begins and ends is hard to define. I find that because of the uniform properties and methods across the base class, Web server controls are much simpler to use than HTML server controls. For instance, across all Web server controls the width property determines the width of the Web server control. This isn’t so with HTML server controls. For instance, with the HtmlInputText control you set the width of the control with the size property, but with the HtmlTextArea you set the width of the control with the Cols property. This can at the very least lead to confusion, which hampers productivity.