ATL is a fast, easy way to both create a COM component in C++ and maintain a small footprint. Use ATL to create a control if you don't need all of the built-in functionality that MFC automatically provides.
MFC allows you to create full applications, ActiveX controls, and active documents. If you have already created a control with MFC, you may want to continue development in MFC. When creating a new control, consider using ATL if you don't need all of MFC's built-in functionality.