f/1.4 to f/1.8 is 3/4 of a stop. Please recall a stop is double the amount of light. If you are shooting in lower light situations, that can be significant. As far as IQ goes, DxOMark.com rates the f/1.4 D and G 50mm Nikon lenses a bit above the f/1.8 versions, but not by a lot. Most lenses perform better a stop or two above wide open aperture, so the f/1.4 lens gives a bit more leeway. I see a used Nikon 50mm f/1.4G on KEH in excellent condition for under $300. Personally, I replaced my Nikon 50mm f/1.4G with a Tamron 45mm f/1.8. It's bigger and bulkier and more expensive, but gets a sharpness rating about 10 points above the Nikon nifty-fiftys. The highest rated glass in the class is Sigma's 50mm f/1.4 Art, which blows everything else away in terms of IQ, but it's a $800 piece of glass compared to the Tamron at $400.