Actually I picked up a 50mm 1.8 new, but they are so cheap. I considered the 1.4, but lots of reviews said the 1.8 was sharper.
When it comes to older lenses, I find more sample to sample variation between 1.4s and 1.8s. I have outstanding 1.4s and so-so 1.8s. I can't speak to the current AF-S versions.
I don't buy new glass on principle . The closest I've come was my 35mm 1.8, which was a store demo.