the 17-55 is better in low light for sure, the f/2.8 is quite useable. IT is a heavy lens, one that is not as easy as a "Carry-around" as the 18-780, which is quite petite in comparison.
If you want low light I would first add a $99 50/1.8 In my opinion f/2.8 is NOT fast enough for indoor shooting with no support and no flash, for the most part. The extra stop and a bit of a 50/1.8 or 85/1.8 will probably make the differencei n low light.
Not that I discourage buying the 17-55 it is afantastic lens, but Nikon erred when not including VR on this lens, that would have made it a low light uber-lens.
Overall the 17-55 will give you quality across the range and across apertures, the pictures really pop.