Most people look at the sensor resolution first, and it is an important factor. While the D3200 has a full 8 more Megapixels, you do need to keep in mind that this will increase the file size when shooting in RAW and fine JPG format quite a lot. This means that you may need larger memory cards and possibly more hard drive space on your computer; factor this in when choosing between the d3200 vs d5100.
Weight is also quite important, especially if you are used to light weight compact digital cameras or entry level DSLRs that are usually very light. Most people also use their cameras mainly on vacations when traveling light is often quite important, making the D3200 the better option.
The screen resolution on the two cameras is the same, which means that there has been a significant upgrade from the D3100 to the D3200. But there is one very unique features on the D5100, which is its movable swivel display. This allows you to fold out the display and turn it almost any direction. One of my earlier compact digital cameras was a Nikon Coolpix 8700, which also had such a display, and I must say that it came in very handy in many situations, e.g. shooting over people or objects, or taking pictures at very low level to the ground.
The frames per second value on both cameras is the same at 4 fps, which is a pretty good feature for a reasonably priced camera. But keep in mind that depending on how fast your memory card is, this speed can quite considerably slow down once the in camera buffer has been exhausted and the camera has to wait until the file has been written to the memory card.
So you might think what is the point in comparing the d3200 vs d5100 when there really are very few differences, and you do have a point. However, the decision process can be even more difficult when there are few difference including the price.
What I bought and why I did so
After a lot of deliberation I decided that the difference in megapixels was a real factor for my usage, as I really want to be able to print out large canvases with, which is better achieved with 24.2 MP. The swivel screen did draw me to the Nikon D5100, which I had been used to with a Nikon Coolpix 8700.
Based on this I decided that it was worth the extra $50 to have a higher resolution and not have a swivel display. At the time that I bought the D3200 the price difference was about $50 and to this day I cannot say that I regret my decision. The higher resolution really comes to use for portraits I take of my kids and A3 size prints look fantastic.
Where Did I Buy The Camera
After shopping around a little bit on the Internet, I decided to go with Amazon. First of all they were among the cheapest, and at the time I think there was only about $10 to $15 between the cheapest ranking websites. My past experience with Amazon has always been good, with orders shipping fast and secure, and most importantly they have free shipping for orders over a certain, quite low, total value.