PHOTOGRAPHERS: Nikkor f/1.4 or f/1.8 lens? Which one?

Hi! I want to get a portrait lens with beautiful bokeh. I've seen these ones, but f/1.4 (fixed focal lenght) is twice more expensive. Why the big difference? Talking about QUALITY, is there a difference using a closer apperture? Better definition or contrast?

I mean: if I take the same photograph with f/5.6, will I see a quality difference using a f/1.4 lens and a f/1.8 lens?

And, another question: If I take the same photograph, USING A 18-105 mm LENS (the one I have that comes with D90) will I see a quality difference? Does it worth to buy a f/1.4 or f/1.8? (not regarding zoom)

Thanks! (sorry for my mistakes in english. It's not my native language)

Nikkor f1.4 and f1.8 is called the fast lens which means you still can get faster shutter speed in the low light condition.
f1.4, f1.8, f2, f2.8, f3.5 are the measurement of the size opening of the aperture or the iris of the lens. The smaller the f-number the bigger the opening size, the bigger the f-number the smaller of the opening size.
when it comes to f1.4, it is the biggest opening you can get of the aperture. That is why it is expansive than f1.8. If you have the budget to buy the f1.4 lens, you go and get it.

18-105 and 50mm1.4? Oh yes.so much different in result.

Jollence Lee