That depends on the size of the element to be tested. The bigger the impactor, the lower the frequency it creates. Low frequency waves are able to...

That depends on the size of the element to be tested. The bigger the impactor, the lower the frequency it creates. Low frequency waves are able to travel further, however its resolution is poorer so they are not as good as to identify small defects. On the other side, the smaller the impactor, the bigger the frequency it creates. High frequency waves do not travel deep, but they have much better resolution, which is useful for detecting small defects. 

If you want to find the backwall of a thick element, you should use the red hammer. If you want to identify small defects in superficial areas, you should use the blue impactor.