An inside mount simply means that the window covering fits neatly inside the window, outlining crisply the inside of the window frame and snugly fitting all around the inside. An inside mount offers the cleanest and most minimal look and can be installed as deep or as shallow as you desire. Accurate measurement is critical as you want the selected window treatment to fit like a dream, just like your favorite jeans that form a perfect shape.
An outside mount covers the entire outside of the window frame and can fit either closely to the outside window frame or you can choose to extend the perimeter of the window because you want to add the illusion of a bigger window or more space or height. You also might have some leftover holes or ugly spots that you want to cover up and by mounting the blind or shade outside the window frame, you can easily create a prettier space. An outside mount can be attached to the molding or beyond. You might have to choose an outside mount if the window is not deep enough for the mounting brackets. If you choose an outside mount, you will have more flexibility and leniency in where and how you mount the shade and any design illusions that you want to create. An inside mount will allow you more light control as the material will cover more of the window with fewer gaps for light seepage and you will have more daylight as the blinds are installed higher.