You said two different specs, you listed the Sigmas aperture range and the canons focal range.

However it looks like there is a Sigma 18-50 f/2.8-4.5, but there isn't a canon 18-50, so I assume you are talking about the Canon 18-55 f/3.5-5.6?

If so, the Sigma is slightly faster throughout its zoom range, so you will have a little better shutter speed overhead, and slightly more narrow dof achievable with the Sigma. But, I would advise against both and suggest Tamrons 17-50 f/2.8 instead, its a constant aperture throughout so its much faster than both, especially at the telephoto end. It has much more dof control and will provide faster auto-focus and shutter speed ability. Because its faster, it will also give you a brighter view through the viewfinder. Its also very well priced. Personally, I would not hesitate to choose it over either canon's kit or sigmas 18-50, hands down.

IF the sigma and canon are your only options, I would opt slightly in favor of the sigma for its slightly faster aperture range, but - Sigma has a history of producing misaligned elements and has a history of poor focusing issues. I would be more enthusiastic about the Sigma if you can see the physical item and make sure its all in good working order first.