I am a beginner in Photography and this would be my first DSLR.
I am confused whether to go for the Nikon D3200 or Nikon D5100. I would be taking mostly portraits and would need a camera which would take better pictures in low light. Also I will go on treks, hill trips often and it should be the best in snapping those landscapes.
Please suggest me the best option to go with.
As far as low light portraits go, these cameras are very very similar. The D5100 is a somewhat older model, and it is reaching the end of it's lifespan. The D3200 is basically brand new. I would take a look at some high ISO comparisons here. This will give you an idea of how closely they do perform at high ISO shots.
Beyond that, the D3200 is a significant boost in megapixels, and if you plan to print very large images(ie landscapes) or blow up small portions of the image, that might make the D3200 beneficial to you. The D5100 does have some higher end features if you think you may need them, such as controls for white balance and exposure bracketing.
Strictly speaking, they should perform very similarly for low light portrait work.
Others have handled the direct answer, but I think a more useful answer is to use more light. Cameras record light. They are really not see-in-the-dark devices.
For portraits, you don't want to have the subject in the sun, its too harsh, but you can take photos on cloudy days, or in the shade and get good results.
You don't have to spend much money, but you should get some lights. Off-camera strobes are a good place to start. The strobist site has tons of information, tutorials, etc. http://strobist.blogspot.com/
I strongly recommend that you not just buy Nikon's latest and greatest strobe and put it on the top of your camera body. Its a better flash than the built in one, but you want it off camera.
There is a decision between manual and automated strobes/flashes, addressing that is a topic that can fill a book.
A few days ago I came across the tests performed by DXO Labs. Note I have no affiliation with then but found their testing methodology, specifically for low light situations interesting at the very least. It seems to me to be a fairly objective measurement based approach that would be helpful when comparing one camera to another.
Looking at their data comparing the D3200 to the D5100 the test results look pretty close to me so I would probably make the decision on price or other features that you care about.
