What would you say your primary concerns were Shaheed? Sharpness? AF? Bokeh?
I researched this myself very recently and found certain lenses were best for certain things. If i remember correctly, these were my findings completely from trawling through online material, i've only ever personally used the 1.4D and only had it a week so haven't had a massive amount of time with it. If anybody has anything to say to the contrary please interject, these aren't opinions based on real life use but from other users online
The Sigma was sharp with pleasant bokeh, but depending on build variations the AF might not be as good but maybe take that with a pinch of salt? I have a Sigma 70-200 2.8 which plenty of people have trouble with AF wise, but after learning to use it properly i find mine to be fine.
The 1.8G was sharpest (out of the Nikons) with good AF and pleasant bokeh but wasn't 1.4.
The 1.4G was sharp to the corners with excellent bokeh (more "clinical" from the reviews i read, but its subjective anyway so you'd have to see for yourself) but not as fast AF wise as some of the others, handled backlit situations better than the D due to lens coatings
The 1.4D had "slight" AF advantages (when used on a decent body, which you have so isn't a problem) and excellent creamy bokeh but not quite as sharp as some of the others, with corners suffering a little.
When i was buying it was all S/H, i discounted the Sigma because of build quality and the pain of having to replace if it wasn't right. I know this may have been shortsighted but I didn't want to get into that debacle (i know i'm contradicting myself a little when i say that as i already own a Sigma lens, but one i could test before buying) I discounted the 1.4G because of the price difference, £3-400 more than the Sigma and the D 1.4s and £600 or so more than a 1.8G. I discounted the 1.8G because like you i wanted a 1.4. The things that turned me on to the 1.4D was it's status as the cream machine and it's supposedly faster AF. Sharpness remained to be seen, but after testing i'm perfectly happy, if the G versions are sharper then they must really be something! I just need more time with it to get used to shooting at such a wide aperture