is there a significant difference in the quality of a sigma lens and a canon lens? like in a 70-300mm lens or any lens for that matter
Sigma's EX line is quite good. Their economy line is so-so and so is that of Canon.
Sigma do have other issues since they sell the same lens (with a different lens mount) for many cameras. They're not always calibrated properly for the brand and the auto focus seems to be inaccurate.
Just to add some clarification... the AF is the issue I had with the 70-200mm. When I got it the AF was terrible, and I mean BAD! I sent it back and got a replacement and it was dead on accurate... on par with any Niikon glass I used. Another shooter I know uses the 30mm 1.4 and had the same issues on his D2xs, he sent it back and the replacement was perfect.
I would pick a comparable Sigma lens over a Nikon for a third the price anyday.
is there a significant difference in the quality of a sigma lens and a canon lens? like in a 70-300mm lens or any lens for that matter
Canon and Sigma both have a wide range of qualities. From cheap to very good. You have to ask about specific lenses.