Canon EF 28mm f/2.8 IS USM or an EF 35 f/2 IS USM on a Canon 700D (crop sensor)? I want to use it for street photography.
I've shot with a 24mm and 35mm on the Sony E-Mount line and I'd recommended at 24mm as it grabs "more" than a 35mm can. It depends a bit on your tastes but 24mm is a good compromise between being too wide (grabbing) too much and grabbing too little, zoomed.
The 24mm is larger than the Sony 20mm pancake but I'd say that if you shoot too-wide you will distort people so keep that in mind.
For Canon, I'd say go for pancake since it makes it smaller but Mirrorless bodies end up being smaller and more discrete.
If I had to pick one of those two, I would go with the 28mm IS.
But, 28mm or 35mm are both too long for close in street photography. For a small and wide lens, I would recommend the new Canon EF-S 24mm 2.8 "pancake" lens, EF 24mm 2.8 IS, or even the older Canon EF 24mm 2.8 for your crop sensor camera.
One of the so called "rules" for street photography is to use a wide angle prime lens and shoot close in to your subjects. Another "rule" is to use a small inconspicuous camera and lens.
(they are not really rules but more like common practices and suggestions)
You might even consider a 20mm lens. Canon and Sigma have some 20mm lenses but they are all rather large.