People have opinions about Sigma/Tamron/Tokina; good and bad. I take photographs as a hobby and sometimes I get asked to do side jobs for money. At the end of the day, I am quite happy with my images. Again, if you are on a budget and looking to save some coin and know what focal length you need and why you need it, get the Sigma from a reputable dealer with a good return policy. If you are not happy with the lens, exchange it, if the next copy is not what you expected, get a refund and save for the Canon.
A camera is a tool, be it Canon, Nikon, Sony, Pentax, etc. I am a customer, of Canon, not because they are better than the other tools, it comes down to how the camera fits in my hand. It is how intuitive the menu is to me. It is the results I get shooting it. I will one day get a Canon 24-70mm F2.8, but for now, I am very happy with the results I get using a Sigma lens on my Canon.