i have the 1.8d and the 1.4 ai. I used the 1.8d for years until i got the 1.4. I love the feel of the focus on the ai and ai-s lenses, much better than the feel of manual focusing the D lenses. I now use the d lens on auto focus cameras.
In my mind, all things being equal I feel the picture quality is slightly better on the 1.8d over the 1.4 ai lens, especially with slide film. with B&W enlargements, the difference is not really noticable until you get up to 11x14 and larger. corners are much better with the 1.8 over the 1.4.
So I guess it depends on what you plan to shoot. if you are shooting landscapes where you need corner to corner sharpness, I have aways prefered the 1.8 over the 1.4, or most of the slower lenses obver the faster lenses. if you are shooting portraits or people or things in the center of the frame at large apertures, like 1.8 to 4 and the corners are out of focus, then i feel that the 1.4 is sharper in the center over the 1.8, but not by a lot.
I also like the fact that you can get the 1.8d lens NEW for $120, the best deal in nikon lenses today.