Why is the nikon d5100 superior to the d3100?

I'm buying the d5100 for Christmas and was simply wondering what makes the price gap. This is for serious photography (not videos or snapshots) and I can finally afford a DSLR of $600 or less. I have researched but am still unsure of what exactly it has over the d3100. Also, why is the d7000 more, if you know? All the real improvements I see in that is the better build and slightly better specs, not worth the $400 gap.

The swivelling screen.

The sensor of the D5100 performs better than the D3100 and the D5100 has an articulated LCD. Some people find this type LCD to have an advantage when shooting macro, from ground level or when holding the camera over their heads in live view

Here is how the sensors perform.

http://www.dxomark.com/...nd3)/Nikon

The D7000 costs more than the D5100 because it is more robust with environmental seals and on in-camera auto-focus motor that allows it to use legacy Nikkor AF lenses and it can meter using legacy Nikkor AI and AIS manual focus lenses.

The D3100 is a super basic entry level DSLR while the D5100 is an entry level DSLR with features that keep the more advanced photographers happy, so its more like an Intermediate type DSLR

D5100 has one stop higher max ISO, 2 more megapixels, bigger and better built body, an articulating screen. Since you said you can afford a $600 DSLR, there's no really no reason to worry about the difrerences, the D3100 is already an amazing camera for the low price!

Here's a DSLR Buying Guide - http://www.smashingcamera.com/which-dslr-camera-to-buy/