I have owned both lenses and currently use the 75-300 IS. The 100 to 300mm is internal focusing (non- rotating front barrel), and this produces much faster focusing. It is also easier to use with a polarizer. The 75 to 300mm is less expensive, except for the IS function, which really does work quite well. Optically, both lenses are somewhat soft at the longer range, which is typical for a non-pro zoom. The 75-300 is fairly good from 75-200 when stopped down, which is easier to do with the IS. If you're looking for really sharp, contrasty slides or negatives for larger prints taken at 300mm, you'll probably need to consider a prime or pro quality zoom. Hope my .02 cents worth is helpful.
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