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

[TheGreatGonzo]

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

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

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[Overread]

Never used either, though it seems that at least The Digital Picture reviews the IS version higher than the MIII
Canon EF 70-300mm f/4-5.6 IS USM Lens Review
Canon EF 75-300mm f/4-5.6 III USM Lens Review

Its not just a drive motor difference, one has the image stabilising feature which is certainly worth considering. In the end the best optical and drive motor differences you can either read about or, more easily, just compare the two lenses you have access to - try them out against each other and take some test shots to compare their sharpness and performance - then keep the one you like and which performs the best for you and sell the other.