I've looked very closely (pixel peeping) at both the 50 1.4 and 50 1.8, and I have never seen enough difference to make a difference in their image quality. The 50 1.4 is better built; but, having said that, I've had a 50 1.4 fail several times in the same way. Each failure has resulted in the lens not autofocusing through it's full range. This was two different lenses, with the result that I'm reluctant to use the lens anymore.
I also own a 50 1.8 (the original metal mount version) that I've had since the late 1980's and used a lot, and it has never failed.
"Nothing exists beyond the edges of the frame."