canon ef 70-300mm f 4-5.6 is usm vs. canon ef 70-300mm f 4-5.6 III usm

[TheGreatGonzo]

As I understand it, the only difference between these two lenses are the type of drive motor? What difference in performance can I expect this to make and, given that I have both (recieved them as gifts), is there a reason to keep one over the other?
Thanks,
Gonzo

 

[Overread20]

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