From a home testing session. Shooting distances were rather close, so I cannot judge performance at infinity on such basis. About 10 lenses were involved; I only quote the 50/1.4 vs 50/1.8 AIS cases
First scene : a kitchen corner in daylight, about 1.0 m
50/1.4 AIS vs. 50/1.8 AIS ___________________________
My 50/1.8 is a recent version, plasticky and with minimum focussing distance greater than 0.6m, so not the best choice, but for the chosen scene, it does not matter. I also got a 50/1.8 AI, which is better in both respects but with damaged coating, so it is used as a paperweight.
At f/2.0, colors and vignetting seem about the same. Center sharpness for the 50/1.4 is better; the 50/1.8 is still marred by some haze. Bokeh is similar in shape and tint (such as green outlining in the background), but the 50/1.8 has more outlining. The 50/1.8 seems to exhibit slightly less LoCA in the foreground; the 50/1.4 has more purple fringing, where the 50/1.8 rather produces blue fringes on shiny parts.
At f/2.8, the overall pictures are hard to tell apart. The 50/1.4 still has the very slightly better center sharpness. The actual FL of the 50/1.8 is also slightly longer. The bokeh is now about the same (both lenses by the way have a 7 straight blade diaphragm, so you won't expect more differences on stopping down).
At f/4.0, differences narrow further down. I still see slightly better center sharpness with the f/1.4 lens.
Second test : simulated night shots (dark room, lots of lit apparatus, crystal vase, computer with backlit keyboard...)
50/1.4 AIS and 50/1.8 AIS _____________________
At f/2, there is less blooming with the 50/1.4. Highlights are outlined in both cases, but they are smoother with the 50/1.4. Highlight shapes are however distinctly heptagonal with the 50/1.4, but still nearly round with the 50/1.8. Colors of the 50/1.8 AIS are more neutral; with the 50/1.4, I observe a pink cast. Keyboard letters are surrounded by a slight haze with the 50/1.8.
At f/2.8, the 50/1.4 still bleeds less: highlights still appear biggerwith the 50/1.8.
Please also note that while I always was pleased with the sharpness of my 50/1.2 AIS (and now with the 50/1.2 AI I bought in addition), it seems that the 1.4 is slightly sharper even at small f/stops (except maybe f/2.0). The 50/1.2 is a most interesting lens, but has more issues (field curvature, probably some focus shift, and sure more CA), so the 1.4 offers a better guarantee of good results, if sharpness is the criterion. Bokeh of the 1.2 is slightly better though.
If you mostly use the 50mm stopped down at 4.0 and beyond, the 50/1.8 is the better choice if distortion is your concern. At such apertures, these 50mm are generally very similar.
My personal feeling with the 50/1.4 is, I do not like the pink haze, so if I intend to do shoot color picture wide open I'll tend to look elsewhere (50/1.2, precisely, or Voigtländer 58/1.4 if coma is no issue).
