See here, image quality doesn't appear much different to D7000, which is either:
1. A very good thing if you like the D7000's IQ but really need the extra resolution.
2. A really good thing if you don't - because it means you can stick with your D7000, if you have one, or if you don't, save yourself about £500 over the D7100 and buy a D7000 instead (but be quick!).
But there are a couple of reasons that might be compelling reasons to buy a D7100 for sports or wildlife, the expansive AF system that covers much of the frame, and the 1.3x crop mode (2x the lens focal length).