Currently I run with a D800E as my primary body and a D610 as my backup/second/smaller body. Prior to the D610 I was using a D7100 in that same backup/second body/smaller body role.
All three bodies use the same battery and same charger, which is a huge plus. My needs for a second body were that it be smaller to fit in the last remaining space in my backpack for landscape use, which is why I don't use another D800 in that role.
The D7100 had nice image quality but ultimately two things led me back to having two FX bodies instead of one FX and one DX body:
1) I simply prefer the viewfinder of the FX bodies.
2) The D610 has cleaner shadows when one has to shove the images around in raw a bit. While the D7100 has IMO the best quality cropped camera low ISO image quality, I still prefer the image quality from the two FX bodies - it's shadow range is not in the same league as the D800E or D610, and of course, the high ISO is better on the D610.
I've not had any issues with the AF on the D610; this was a major concern because I had show-stopping major issues with the 39pt AF system in the D7000 I had a year or so ago that I truly hated.
This all being said, the D7100 has very nice image quality and if you want a DX body and shoot mostly at lower ISO, you will still be able to get a lot of quality out of the body; you just might have to be more careful of underexposure since you don't have the range in the shadows.