With both the 1.8 and 1.4 lenses, the earlier versions tended to be sharper in the center. Later versions not quite as sharp in the center, but the corners are sharper. Later versions of both lenses also are contrastier, less flare prone and SLIGHTLY more flat-fielded.
If stopped down in the F5.6 to F8 realm, the 1.8 and 1.4 render nearly identical images. My controlled testing reveals no advantage one to the other.
I personally reach for the 1.4 because of how the bokeh renders. It's just a little more artsy. Mine are all of the last version made.