Whether you need a lens with IBIS has a lot to do with the conditions under which you shoot, and your technique.
Focal Length and Shutter Speed:
Conventional wisdom suggests that the minimum shutter speed to get a steady shot when hand-holding a lens without stabilization is 1/focal length. However, you are using a crop sensor camera and that demands an extra factor so, in your case, a safer value would be 1/ (2x FL). So, you would be looking for shutter speeds in a range from 1/40 to 1/100sec for the Sigma 18-50 and 1/20- 1/40 for the 10-18 lenses. With stabilization you could do without the 2x factor in that equation.
Technique:
A lot of people coming from a cell phone tend to hold a camera in the same way, with arms extended, looking at the LCD in the rear. This is not a good technique because dedicated cameras are heavier and have longer physical focal lengths. The correct way to hold a camera for still photography is as shown in the image below:
You want to use the viewfinder with the camera pressed to your brow. The heel of the left hand under the body, with the fingers curled around the lens from beneath to manage the zoom. The right hand is free to gently press the controls and shutter. The arms are tucked tight to the torso. Thus, with the eye to the camera, your body creates a tripod effect and that can compensate for the lack of Image Stabilization.
Aperture:
The Sigma offers one more stop of light than the Canon, which can be expressed either in terms of being able to use a faster shutter speed, or lower ISO.
Light:
The lower the light, the slower the shutter speed, wider the aperture or higher the ISO must be. It's always a play-off.
As always, much depends on what you are prepared to invest, but I would note that the build of the Sigma is considered superior with such differences as a metal lens mount and better engineered plastics - they just feel a lot better quality. The down side is that you pay more, but most consider you get what you pay for.
