Nikon D3200 or D5100?
Download the raw images from both and process them on your computer and see exactly what the differences are, thats what I do. You can get the files from the comparisons in the camera reviews. IMO the 24mp files have more detail at low ISO and at high ISOs when downsized match the 16mp files. If you need the larger files get the D3200 if you don't get the D5100. Both have much better sensors than the older sensors on the D90 / D300 /D5000. At base ISO I don't see much difference between the D600 and D3200 in resolution (D600 is slightly sharper but a little sharpening brings the D3200 very close at 100% viewing). If price is not an issue you might wait until January and order the D5200 which also has an upgraded AF.
When testing the different ISO files also try bringing the exposure up 1-4 stops to see how much the files can take before noise becomes too much of an issue. The D600 / D800 are champs at this.