The D3200 improves on the D3100 in terms of its HD video capabilities. The D3100 was only capable of recording Full HD 1080p at 24fps, however the newer D3200 shoots at 24,25 and 30fps. On the D3100 the movie-rec button was found at the rear of the camera.
Another improvement on the D3200 over the D3100 is the addition of a 3.5mm microphone socket ready to support an external mic. This can be used with microphones such as Nikon's ME-1. It's worth noting that the D3100 does not feature a 3.5mm mic port, meaning if you're looking to take better of control of audio, the D3200 makes the better choice. The D3200's movie-rec button is also found on the top plate beside the info button rather than at the rear of the camera.