The 50mm f/1.4 lets in 65% more light than the 1.8, so it is better for night photography. If you use it for portraits, it can blur the background more. It does come with a price though, at f/1.4 the depth of field becomes very small. If you take a head shot, and focus on the eyes, you will notice that the tip of the nose is out of focus. Because of this you will often have to go to f/1.8 of f/2 anyway. It is also extremely difficult to shoot moving things, like children at f/1.4, it is worth it though, no other lens lets you shoot without flash in such low light(unless you spend 10.000 dollar on a 35mm f/1.4, which performs about the same).

Apart from this the lenses are identical and give the same (extremely good) quality of photo's. The 50 mm is the favorite lens of many professionals. Besides night, party and portrait, it is also a great lens for traveling because it is small and light.

If the small difference between the two lenses is worth the difference in price is something that everyone needs to decide for themselves. I am very happy I went for the 1.4 almost 10 years ago. A good lens should last at least 10 to 20 years, and I use it a lot. I expected to use it about 5% of the time, but I still shoot about 95% of my photo's with it, even though I have 3 more nice lenses.

Forget about the D lenses, they don't have an autofocus motor, and use the one in the camera body. Since your body does not have one, that won't work very well.