JPEGs are clean through ISO 200, good though ISO 1600, and usable to ISO 6400. The extra sharpness delivered by the OLPF-free sensor helps maintain sharpness as ISO sensitivity rises.
The D7100's images are a hair sharper than the D5200's, but much of that can be compensated for by using a better lens on the D5200 (the D7100 comes with a better kit lens). Once you equalize for that, only pixel peepers will really note a difference at low ISO sensitivities.
Even as low as ISO 800 I could get better results processing the raw images than using the default JPEG settings. (JPEG top; raw bottom. I didn't attempt to preserve the red tint.)
(1/50 sec, f4.5, spot metering, ISO 800, AWB, 18-105mm lens at 40mm)
Nikon pushes the hues and contrast a little too much with its default Picture Control settings. (You can see the red hue shift here, but this example doesn't show how the contrast boost clips detail.)
(1/100 sec, f5, matrix metering, ISO 100, AWB, 18-105mm lens at 48mm. Images rotated 90 degrees to fit.)
