Here's the thing about comparing two very different lenses like a 50mm f/1.8 prime and a 17-50mm f/2.8 zoom:
They're made for two different purposes.
Yes, there is a bit of overlap at 50mm and apertures f/2.8 and narrower (higher f-number). But the reason for buying a 50mm f/1.8 over a zoom that includes 50mm is to get the larger aperture and the better image quality that a prime usually gives over a zoom. (I'm sure there are examples somewhere of a very expensive, wide aperture zoom with a fairly limited zoom range that does better than a really cheap prime in the same focal length range, but that's a rare exception. Even $2,000+ zooms with 3X focal length ranges usually fall just a bit short of the same image quality or, in some cases, equal a prime priced at $300-500 for the same mount and sensor size.)
Looking at the broader picture, zooms are made to be more flexible at the expense of image quality. Primes are made to be better optically at the expense of flexibility.
What you need to ask yourself (instead of a bunch of strangers on the internet that have no idea what you want to do with another lens beyond what you already have) is, "Which do I need more, the focal length range of a 17-50mm zoom with an f/2.8 constant aperture and slightly better image quality than what my 18-55mm f/3.5-5.6 lens is currently giving me OR the wider aperture and moderately better image quality of a 50mm f/1.8 prime?"
Keep in mind the following:
- 50mm is pretty tight on an APS-C camera. To take photos of groups of people, you'll need to back up more than many indoor homes or apartments will allow. For an APS-C camera, a single prime like a 35mm f/2, or even a 28mm f/2.8 'pancake' may be a better choice for many photographers.
- Getting a lens that gives better image quality does not guarantee you'll take better quality images. Image quality is about how well the hardware performs. Quality images are about how well the photographer sees and controls light, how well the photographer composes, and how well the photographer exposes and post processes images. A better lens won't instantly make you a better photographer.
- The Sigma 17-50mm f/2.8 is in the same "class" as the Tamron SP 17-50mm f/2.8 and the Canon EF 17-55mm f/2.8. In my experience the Canon is clearly the better optical performer. The Tamron also seems to edge out the Sigma a little bit. (Full disclosure: I've owned the non-VR version of the Tamron SP 17-50mm f/2.8 Di II since late 2008. I still occasionally use it when I need a 'normal' zoom on an APS-C body, but I generally now use FF bodies for all but some telephoto work.)