Comparing the Canon EF 35mm f/2 IS USM Lens with the Sigma 35mm f/1.4 DG HSM Lens - which to choose:
If you need the f/1.4 aperture, for the shallow depth of field or for the faster shutter speeds it makes available (key to stopping action in low light), your decision is made. You need the Sigma 35 f/1.4.
If you need Image Stabilization, the Canon 35 f/2 IS is of course the right choice.
If f/2 is wide enough for you and you don't need IS, the decision becomes a bit more complicated. Here are some comparisons to help with that decision process.
The Sigma of course rules all comparisons at f/1.4 - and turns in remarkable performance at f/1.4 compared to other f/1.4 lenses.
The Canon joins the comparison at f/2. At f/2, you can expect the Sigma to be noticeably sharper in the center of the image and the Canon to have a slight sharpness edge closer to the corners - where the Canon shows more vignetting. The sharpness comparison at f/2.8 has these lenses performing more similarly in the center (both excellent). The Sigma retains a slight center advantage and the Canon holds the peripheral edge. Beyond f/2.8, both lenses are razor sharp with the Canon retaining slightly better full frame corner sharpness.
The Sigma has about 1/2 as much vignetting at comparable apertures until stopped down to about f/4 where the Canon trails the Sigma by a very small amount through f/16. The Sigma has slightly less flare until the aperture narrows to f/8. The Canon then has noticeably less flare through f/16. Neither lens has significant distortion. These two lenses deliver similar (good) bokeh at comparable apertures.
The Canon weighs 1/2 as much, is smaller and focuses closer with a higher MM (0.24x vs. 0.19x). The Canon is modestly less expensive at this time.
Here are the full reviews for these lenses: Sigma 35mm f/1.4 DG HSM Lens Review Canon EF 35mm f/2 IS USM Lens Review