I am not sure which Canon lens you are looking at:
My catalog shows:
Canon EF 75-300mm f/4-5.6 III .....$160
Canon EF 75-300mm f/4-5.6 IS USM...$190
Of the two, you should only consider the second one.
I found a review on it.
The Sigma catalog shows:
Sigma 70-300mm f/4-5.6 DG Macro......$140
Sigma 70-300mm f/4-5.6 APO DG Macro..$219
I found a review on the second one, but not the first one.
With long zoom lenses in this price range, you will typically find that the image quality out to 200mm is quite good.
But image quality out to 300mm is not so good.
The Sigma lens is sharper (in the middle part of the lens) out to 300mm than the Canon lens.
Both are not sharp near the edges out to 300mm.
Here are the reviews:
Okay, I have had a Digital Rebel XT for almost a year now. I have the kit lens and one prime lens for portraits. I'm going to the fiftieth anniversary of the Central High/Little Rock Nine crisis later this month, and I wanted to buy a cheap telephoto lens to take pictures from my crappy seat. Plus, I need to round out my lens collection.
The two lenses I'm thinking of:
Canon EF 75-300mm f/4-5.6 III USM
Sigma 70-300mm f/4-5.6 DG
The price difference isn't that big of a deal for me.
Usually I would spring for the Canon, but I've read some bad reviews.