In Nikon, the 50mm f/1.8 AF or AF-D and the 50mm f/1.4 AF or AF-D lenses have been unchanged for years, so if you're considering the Nikkor f/1.8 versus the Nikkor 1.4 in the AF-D versions, there really is not a lot of quality difference between the two lens speeds. Unless you buy the newest 50mm f/1.4 AF-S G Nikkor, which has one problem that might make it less-suitable for low-light work, and that is its very slow autofocusing. There's not "that" much difference between the 1.8 and 1.4 Nikkor lenses in the screwdriver focusing designs.
In many low-light situations, capturing 'better images' depends more of focusing speed and focusing accuracy and focusing reliability, than on ultimate optical quality. Most image shot of 3-D objects in low light annot be rendered all that well at f/1.4 or at f/1.8,and you end up shooting at f/2.8 or so in order to have adequate depth of field to render subjects well, so the difference between the 1.4 and 1.8 Nikkor lenses is quite small. c cloudwalker's advice is good--look for a used 50mm lens....the last 50/1.8 I bought I payed $40 for at a pawn shop that sells a lot of camera goods.
In Canon, the difference in optics, design, build quality, and bokeh, as well as AF speed, AF loudness, and AF reliability between the 50 1.8 EF-II and the 50/1.4 EF lens is very substantial; the 50mm f/1.4 Canon EF is an order of magnitude better than the 1.8 EF-II model in almost all categories.