Aloha, you didn't mention whether or not you are looking at the AF lenses. Build quality AF1.4 vs AF1.8, 1.4 is slightly more robust, the 1.8 feels and is mostly plastic, but is surprisingly tough in use.
I prefer the 1.8, MF, Ai version. I have many, many 50's and that one to me has the best balance of speed, sharpness, bokeh, price and build quality. The 1.8 is a 6 element design, versus the 1.4 7 elements, I just prefer the lower level of distortion of the 1.8.
If you plan on using the lens extensively for low light you may want to get the 1.4, but I've done plenty of low light work with the 1.8 as well as the Nikkor f2, which recently has started to creep back into my lens rotation. On screen 1.8 and 1.4 look nearly the same, the 1.2 seems a smidge brighter than the 1.8.
The addicting thing is that 50's can be found for fairly cheap, so its kinda fun to collect a few and see for yourself which ones you like. The old Nikkor S 1.4 has a nice character in B&W, at about f4. Good luck and Aloha