One of the key differences between the D3100 and the D3200 are the sensors found inside. The Nikon D3100 features a 14.2Mp CMOS sensor whereas the newer Nikon D3200 features a 24.2Mp chip.
Both sensors are APS-C size so you'll have to bear in mind the 1.5x multiplication crop factor when attaching lenses. Both the D3100 and D3200 support Nikon's range of DX lenses and the 18-55mm kit lens is equivalent to 27-82.5mm in 35mm terms.
To achieve the D3200's higher resolution Nikon has squeezed more photosites (pixels) on the same sized chip.
Of the two cameras the D3200 offers the greater sensitivity range. The Nikon D3100 features an ISO range that runs from 100-3200 (with the option of expanding this to ISO 6400 and 12,800 in the Hi 1 and Hi 2 settings. In comparison the D3200 has a native ISO range of 100-6400 with the opportunity to expand this to 12,800.
To view our image samples from using the Nikon D3100 click here.
To view our image samples from using the Nikon D3200 click here.