eduardo_frances posted a reply:
the advantage goes for the 75-300mm USM for two reasons
It has USM -ultrasonic motor- it will focus somewhat faster, and quality control, Sigma and Tamron have some serious with sample variation -front focus, back focus, de centered elements, etc-
Even when Sigma and Tamron want to label those lenses as macro they aren't true macro lenses.
My experience with sigma lenses is very bad i worked at an studio where they had the Sigma 10-20mm and 18-50mm f/2.8 EX. the 10-20mm vigneting was weirdly more darker on the upper corner and had a very ugly geometric distortion - it looked like the step of a ladder in the borders, not a gradual spherical distortion- and the 18-50mm misfocused all the time, and at f/2.8 it was ugly soft....
Hope it helps :)
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