Is D3100 much different image quality than D7000?
What type of shooting?
What type of printing from the output?
The basic peice you might want to consider is the D7000 being the highest density sensor in Nikon's lineup, it requires some care to get the most from it.
Sturdier support or shooting technique.
Faster shutter speeds to name a few.
If you are a general shooter that doesn't have a desire to explore the craft, and all you wish to do is record family events, and dabble in photography, the D3100 would be my choice. It requires less considerations to get reasonable results in average use.
If your looking to really dig into photography and don't mind improving and investing in yourself to hone the craft, the D7000 would probably be more suited to your use. It's a better camera to grow with, but it could get frustrating if you decide later your just interested in snaps.
Both are very capable. Don't read into my comments that you couldn't excel or grow with a D3100. You would just eventually hit limits with it that the D7000 would solve as you advanced in the craft.
“There is only you and your camera. The limitations in your photography are in yourself, for what we see is what we are.”
~ Ernst Haas