In my view the size difference between the three 35mm primes is not a real issue, at least not for me. Yes, the 1.4L and the Sigma are both a bit bigger than the 2.0, but still compact. If size is a real issue for you, Iīd go with the 40mm pancake.
Since you already have the 24-70 2.8, I honestly donīt believe you will use a 35mm f2.0 very much. I know I would not. I have the the 24-70 f2.8L II and the Sigma 35mm f1.4 and the Sigma is being used because it gives you the extra aperture. I also like the shallower DOF and nice bokeh. The color and especially sharpness of that lens is also exceptionally good for the price. I normally have things moving in the images I use it for, which is pretty much what you describe what you do, so IS does not help me very much. But I donīt do video, which clearly would benefit from IS.

Good luck!
Eldar