I have the d3200 and it is a great camera! I love the guide that helps me take professional looking photos as a beginner. I cannot speak for the d3100, but the 3200 has more megapixels and a guide for learning. My photos are amazing. I did purchase an additional 70-300 mm lens, but even the one it comes with is great. I just wanted to take pictures of nature that was further away.

Both are good cameras as far as construction, weight and functionality. Either will give great results. Mega-pixels over 12 to 16 are personal preference moslty. The larger the megapixel the larger the picture you can print with clairty. Decent ISO range on both. Both are capable of HD 1080p video recording. The D3200 can shoot 4 frames per second in continuous mode. I am a nikon shooter and have been for years so I tend to lean that way. The biggest difference will be in the lens quality. Nikon has superior glass (in my opinion). I bought the D3200 as a back up for my D7000. I am very pleased with the output quality. It has several manual and auto modes (fewer than the D7000 of course). Its a great starter camera for learning to shoot in manual mode.

Nikon does not produce or manufacture a battery grip for either the Nikon D3100 or D3200. There are aftermarket grips that work well though and use up to 2 batteries. The only caveat is there is a small external cord that connects from the battery grip to the camera to use the vertical battery pack shutter.

Yes it comes with the same lens as the D3100, D5000, D5100 camera. 18-55mm DX VR lens

They may, depending on the camera settings and your initial point of reference. The D3200 is certainly going to use up your SD card faster than most cameras, all else being equal.