Have not heard good things about the 75-300mm USM III. Pictures are really soft and it will feel cheap.
"If you care about great image quality and sharp photos, the Canon EF 75-300mm f/4-5.6 III USM Lens is not for you.
And Photoshop cannot enhance details that are not there."
Just from that alone I would lean towards the 100-300mm despite the shorter zoom range and higher f-value. If you are going to be doing most of your shooting at nearer 300mm then even more reason to get the 100-300mm because the f-values are the same and it is a much better quality lens. At max zoom it will perform much better than the 75-300. The £50 more it costs is worth it imho.
Of the two you are looking at certainly the 100-300mm is the better lens but it might be worth thinking about it a bit more and seeing if there are any other lenses that fit the bill. Another option would be looking at refurbished/used lenses of a similar focal range but further up the price ladder (if they were new).
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