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.

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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:

http://www.photozone.de/8Reviews/lenses/canon_75300_456is/index.htm

http://www.photozone.de/8Reviews/lenses/sigma_70300_456/index.htm

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