Optical Flow Sensor for Mobile Robots

Hi my name is Esteban Falconí, maintenance supervision assitant of F.V. Área Andina S.A. we are developing and AGV and it seems that you have the availability of using besides magnetic sensors , optical flow sensor for the guidance of the robot, please can you explain which one are the main diference for the same aplications or if there are some advantages betwen using both types of technology or just one. 
best regards 

maintenance supervision assistantmaintenance supervision assistantmaintenance supervision assistant