I would definitely prefer a Zoom lens over a single focal length lens.however, both of the lenses in question are extremely large for what they are, in terms of focal length. The 50 from Sigma is huge--One of the largest 50 mm lens is you'll ever see. The zoom is also very large.
Four street work, I think a smaller lens works better and draws less attention. The sigma 50 mm art is not designed to be a candid lens.
I cant speak for the 50, but I have the 18-35 (for Canon) and it's my #1 lens for portaits. (I do have a 7dMKII though so I am shooting crop as opposed to FF). Last time I checked, it was rated extremely well on the typical compare charts.
After all I got my hands on the Sigma 18-35mm F 1.8 ART today. It has such a beautiful razer sharp image quality and artistic bokeh I can only recommend it so far.