Back to the question.
If cost is it(correction: should be NOT, not it) a big problem, I will suggest go with the F/1.4 version.
In general, if you take 2 photos, one with F/1.8 set at F/1.8 and the other one with F/1.4 set at F/1.8, the F/1.4 version usually gives you a sharper picture.
If you do a lot of low light photography without using flash, F/1.4 of course is better.
For me, I have the Canon F/1.8 MK1. If I have more money to spend, I will go with the F/1.4. But now ... I am fine with my F/1.8 lens.
In Canon version, the F/1.4 focus faster and it has more shutter blades than the F/1.8. (The blur background type photos is more creamy with the F/1.4) I am not too sure about the Nikon side though. Of course, you can search around and see if it is the same for the Nikon.