How often do you shoot at f/1.8?
How often would you shoot at f/1.4?
The difference of viewfinder brightness, unless you are shooting in extreme darkness, is probably not worth the extra expense if you already own the f/1.8 lens.
As far as optical performance, if most of your images are shot in "normal" daylight, with the lens stopped down, I doublt if you'll see improved image quality in the final print or transparency. If you are shooting at the edge of extreme lighting conditions wide open, perhaps the f/1.4 will improve image quality slightly when shooting wide-open. You'll see the slight improvement in extreme enlargements.
Personally, save your money to buy other lenses.