If you exclude the small difference in maximum aperture, and are comparing performance stopped down a stop or two, the Zuiko 50mm 1.4 and 1.8 lenses perform very similarly - any difference in performance is more likely due to sample variation then it is due to design differences.
If you do measure those things that can be measured on an optical bench, you will probably find that one will be slightly better at one f/stop, while the other will be slightly better art another.
The later versions have better coatings.
The f/1.4 version gives you a brighter viewfinder. The f/1.8 version is very compact.