I was looking at Nikons when deciding what to buy. To be honest, you'll probably be happy with whatever you get, but there are a few things to take into consideration. Don't buy the marketing BS with megapixels. You won't find manufacturers touting resolutions for DSLRs like you do with compact cameras, because unless you are doing pro shoots, the sensor resolution really doesn't matter. What matters is that the camera has a great range of lenses, and you enjoy using it.
I tried quite a few cameras out until I got it down to the Pentax K-r and the Nikon D3200. You will find
this website very useful, it will objectively compare any two cameras you want. In the end, the two things that did it for me:
Pentax has an amazing lens lineup, they have only ever had one mounting system and you can use ANY Pentax lens. Nikon has several different systems and so if you upgrade your camera, you run the risk of loosing a large investment.
The K-r also has a proximity sensor which shuts the LCD off when you put your eye to the viewfinder. This doesn't sound that big of a deal, but it's actually surprisingly distracting having the LCD blaring in your vision when you're trying to frame a shot.
This is not me pushing Pentax on you, I merely want to point out that there is more to comparing cameras than specs alone! Nikon make some amazing cameras, so hopefully you'll be chuffed whichever one you choose. Read some magazines and ask around a bit - don't rush into making a decision because it'll be a bloody expensive mistake if you pick the wrong one!
http://snapsort.com/compare/Nikon-D5100-vs-Nikon_D3100
One thing I love about the current Nikon lineup is they all do 1080p video.