Some say (assuming you are talking Canon) that the 1.4 has a nicer bokeh (out of focus effect).
There's a bit of light gathering difference, but the DOF at 1.4 is extremely shallow and lenses aren't at their best wide open.
In most cases, Canon especially, the 1.4 is a better made lens. The Canon 1.8 has a plastic mount and the nastiest manual focus ring in the world. However, it's very cheap and is capable of excellent sharpness and optical accuracy. I bought one, and use it all the time.
Personally I'd get the 1.8 (I did!) and a cheap zoom lens, for the same price as the 1.4.