If you find teh 24-70 is a little too short, or that at f/2.8 it is too slow, then an 85mm might be the ticket. I've owned the 85mm f/1.8D and 1.4D Nikkors, and the Sigma 1.4. I kept and still use the Sigma. It's my favorite portrait lens. I have not used either of the G Nikkors.
In terms of price, the Sigma is less than either 1.4D Nikkors, and about twice as much as the 1.8 Nikkors.
The next question, of course, is whether what you are shooting will work the narrow depth of field of either f/1.4 or f/1.8?
You can overcome a slow lens with higher ISO, but aperture is forever.