You do not say what pictures you want to take with this lenson and that is very important.
I do not know about the relative optical quality of these lenses but I am sure someone else will tell you. That could also have a big bearing on what you do.
What do you use the camera for. Do you find what you have now forces you to use too low a shutter speed. It must really depend on that answer if you need that extra speed. 1.8 to 1.4 is not that much increase. Can you turn the iso up ?
I was going to suggest that the 30 mm would give you a wider angle option over what you have now. I have always worked on the feeling that options are good
but again do you need it for your photography.