I've had both the f/1.4 and f/1.8 versions of the Nikon 50mm and I'd say that if you have to ask, just get the f/1.8. If you find you really need the f/1.4 later, sell your 1.8 to somebody else just starting for hardly less than you bought it for new (or the SAME price if you bought used, lol) and go upgrade to the f/1.4.
In my experience, the build quality is the same cheesy plastic Chinese build quality. The 1.8 is a little sharper wide open, more resistant to contrast wash out when shooting into light sources, and is overall easier to shoot with. The 1.4 is a bit softer wide open, the contrast degrades more when shooting into light sources, and getting anything in focus at f/1.4 is a real chore and requires far more attention to composition and pushes your AF system to the max. Overall I like the optical performance of the 1.8 better, but I shoot with the 50mm when I need speed, and when I need speed, f/1.8 is still "slow", but f/1.4 will get the job done.
The 1.4 lets me pull off stuff like this, along with the silent shutter of my D40. Manual focus, iso1600, 1/25s, wide open at f/1.4.
The 1.8 would have put me down at 1/15s which would have been even more difficult to hand hold well. I would have needed iso3200 to make up the difference, but the added noise would have ruined the "peaceful sleeping tranquility" effect of the photo. That above is literally straight off the camera with no processing.
That's extreme. For "normal" use the f/1.8 is fine and actually probably a better choice, a bit better optically in my experience, one-third the cost, and far easier to shoot with, which is why I say if you have to ask, just get the f/1.8.