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.