Wide angle lenses are often favored for landscape and indoors, because it's typically difficult to "zoom out" with your feet in both cases. The f/1.8 would definitely have an advantage if your indoor parties are dimly lit and people are moving around, but besides that the OS on the Sigma can make up for the narrower aperture in many situations. In isolation, I would recommend the 17-50 2.8 over the 50 1.8 for your stated use cases.

One wrinkle is that you already have a lens that covers almost that exact focal range, albeit with a narrower aperture and no stabilization. Basically you will be replacing your current lens, rather than augmenting it. The primary advantage of the 50mm 1.8 over either your current lens or the Sigma is that you can get narrower DOF (and the attendant nicer bokeh) and a sharper image (the Sigma should improve on both counts over your kit lens too, just not as much as the 50.)

Then of course there's the cost difference. The relative value you place on all that - and therefore which lens is more suitable - I'll let you decide, but hopefully that gives you better view of the pros/cons.