Hello again..well, I got the new d7100 body yeste... (
Let's get a few specifics straight:
1. Nikon AF-S 35mm f/1.8G DX, $200
2. Nikon AF-S 35mm f/1.8G, $600
3. Nikon AF-S Nikkor 50mm f/1.8G, $220
4. Nikon AF 50mm f/1.8D, $120
5. Nikon AF-S 50mm f/1.8G Special Edition, $280
* The two current "nifty-fifty" lenses are #3 and #1 ("nifty-fifty-DX"). The 35mm DX has the field of view of a 52mm FX lens.
* Lens #2 is more expensive because it is not
a nifty-fifty, it is a wide-angle lens for an FX camera. Covering the FX sensor is more expensive than covering the DX sensor, especially for wide-angle lenses.
* Lens #4 is the old "nifty-fifty" which does not have a focus motor built in, so it is cheaper. It is an "AF" lens, a lens with the focus motor is labeled "AF-S" like the other four on the list. Your D7100 has a focus motor, so you can use #4 in almost the same way as #3, saving $100. The significant difference is that you can have #3 in AF mode but focus manually, while you have to switch #4 from A to M to focus manually.
* Lens #5 is an overpriced lens to go with the similarly overpriced Df.
As far as whether you want the 35mm (#1) or 50mm (#3/4) lens, I think the 50mm seems to meet your needs better. One factor for shallow depth of field ("blur in the background" ) is having a larger distance to the subject, which will happen more with the 50mm. The 50mm will also be better than the 35mm for portraits.
I will throw out one other option:
6. Nikon AF-S 85mm f/1.8G, $500
Lens #6 is more expensive because it is a longer focal length and still f/1.8. It also has excellent image quality, one of the best Nikon lenses. And it has excellent bokeh (good "blur" ).
Editorializing: The 35mm prime is a "normal" lens, but I don't want "normal" pictures. The 85mm, especially on a D7100, would be different, in a good way. :-)