Zuiko 50/1.8, six elements in four groups; Zuiko 50/1.4 seven elements in six groups. So, from the standpoint of contrast and flare control the 1.8 will perform better than the 1.4 because it has fewer internal surfaces and smaller diameter elements - 1.8 has six internal surfaces, the 1.4 has ten.
All other things being equal, the slower lens will have greater apparent sharpness-"crispness" if you will.