I presume you mean ISO 12233?
Like any f1.4 and f2.0 lenses of the same focal length, there is exactly one stop difference between them - so the larger aperture lens will have the option of letting you shoot at the same action stopping shutter speed and ISO in half the given light, and it will also let you blur out the background twice as much. However, the Canon 35/2 does have a four stop IS system, so if you're shooting stationary subjects in low light without a tripod, the Canon 35/2 will let you get away with 1/8th of the light (three stops lower).
To compare DoF to 50m lenses is difficult as you'll no doubt be closer up for the same framing with a 35, but generally speaking a 35/1.4 is about the same in terms of DoF as a 50/2 lens, and a 35/2 is about the same as a 50/2.8.
And not one of these four lenses (Sigma 35/1.4, Canon 35/2 IS, Canon 50/1.2 L or Canon 50/1.4) is sealed. As Neuro said, 85 to 135 is much better for portraits. 85L and 135L are probably top of the list - again, neither of which are sealed.
ps - It might be worth watching this video Ray2021 dug up: http://www.youtube.com/watch?v=vC0yvuXnRN4#ws