Thoughts on sigma 17-50mm?

I'm in search for buying new lens but don't know if its best to choose a fixed/prime lens like nikon 35 mm or choose one like sigma 17-50mm. What troubles me its that, if buying a fixed lens i will always have the limit of 35mm but if choosing a lens like sigma 17-50mm i will have thaechange of shooting in variety of focal lengths(from 17-50). I want lens with wide aperture
What i will be shooting is street maybe some distance candid photos and some portraits.

Will the sigma 17-50mm work for all these kinds of photography?
Or what about 24-70?

Much appreciate your answers
I have Nikon d3100-crop sensor

For crop-frame cameras a 17 to 55 is an excellent standard zoom, good for weddings, portraits, landscdapes. The extra reach of 70mm is not much more - if distance shots are needed then you need at least 85mm. 24 to 70 is designed as a standard zoom for full-frame cameras.