Is it better a AF-NIKKOR 50MM F/1.8 D?

Or a Nikon Nikkor 50mm f/1.4 G AF-S?
Do you know the difference between the two?

The bigger f/1.4 aperture is the obvious difference. The AF-S means that it will auto-focus with All Nikon DSLRs, while the older D lens will not. The question is, are these features important enough to you that you will pay twice as much?

An f1.8D will not autofocus on anything below a D7000 or D90

There's slight difference in aperture. The 1.4 is build better and motorized. Optically it's pretty much the same once you get to f4.