Are you planning to use one body + one lens combo? In that scenario, for general use, IMHO 17mm focal length is more suitable than 45mm. Easier to use (in terms of working distance needed) for scenery, street photography, full-body portraits, and for taking pictures indoor For this usage pattern, might also want to consider 20mm or 25mm lenses as alternatives.
Having said that, it's better to carefully consider your shooting style and the kinds of problems you usually face with your current gear For example, I myself started with 12-50mm, then bought a 45mm f1.8 because I don't mind carrying two lenses and the 45mm prime is much faster than 12-50mm at 45mm. In my case a 17mm f2.8 won't provide significant enough benefit over the 12-50mm lens at 17mm (I eventually bought a 17mm f1.8 instead).