There is no functional difference between the ps/2 and usb option on your mouse. They simply include the adapter if your computer doesn't have usb ports, or the operating system does not support usb (such as Windows NT 4). This allows the company to affordably allow its product to be used by everyone. The 200hz it is refering to is probably the frequency of the optical mouse. USB is faster/ new than PS/2, which means if you use the USB funcion of the mouse, you will have better response from the mouse. I doubt you, or anybody would be able to tell the difference, however when doing extreme graphics work, and some hard core gamers, the need for the "purest" input possible requires that usb be used. Put simply is the difference between a hair width, and half a hair width.