I've owned half a dozen of Sigma lenses in the past. Out of 6 lenses, only 1 worked properly out of the box. The others went back 1-5 times for warranty repair. Sure, the service support you get from Gentec is top notch but it's such a pita, especially if you need your equipment to work when you call for it. Sigma lenses (30 1.4, 50 1.4, 10-20mm, 12-24mm, 18-50 2.8) are unreliable from my experience (these issues are well documented across the web)... decentered images, poor focus (back/front), AF motors dying for no reason, aperture blades getting stuck and the list goes on...
Imagine an equipment/lens breaking down on you during a vital shoot (worse, all your back-up is Sigma too)... what would you do? Probably upgrade to a Canon
If you can afford to take the risk - then go Sigma.
If your budget doesn't allow it, get a 18-55mm kit lens