The Zuikoholics (of which I am a member) will tell you that the best 50/1.8 is the last version (says "Made in Japan" along the front of the lens) and the best 50/1.4 is those with a serial number higher than 1.1 million (I've never seen anything saying exactly when the last edition came out).

Personally, when my 50/1.8 started to go (sticky aperture blades) and wasn't worth being fixed I picked up a 50/1.4 and noticed it was much sharper. I don't know what the serial number of the 50/1.8 was but the 1.4 is in the low 1 millions so I suspect that it's the last version or at least the 2nd last version while my 50/1.8 was probably an earlier version.

I had a chance to compare a 50/1.8 and 1.4 in a camera shop several years earlier and I remember not noticing much difference in the brightness in the finder. But, with the difference in the lenses that I had I did find the 1.4 was easier to focus but I suspect this was due to it being a better lens (it's really sharp).