It depends on what you're planning to do... The 5100 is a very capable entry level DSLR and the D7000 is the most advanced amateur/pro camera Nikon makes today so it is hard to suggest what would be the best for you. Since you've had practice with the D5100, you should know what you're missing or would like to have or if perhaps the D5100 is more than what you would want in a camera... its all relevant to your needs, desires and knowledge.
I know quite a few pro photographers using D7000's and D300s cameras and seem to do quite well.
Here is a link to Eric Diller's home page.. the guy uses a D300 and a D7000 and his pictures are sensational, just look at his bird collection

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99% of the photographers will tell you to spend your money on good glass rather than a camera as even an entry level can get splendid results when used correctly with the right glass under the right circumstances. The main problem with good glass is the price

... which most people have a hard time justifying but once they taste it, they will never look at any other glass but pro.
To be setup for most situations you will need a wide angle lens and a telephoto lens along with a flash and it all boils down to how much you're willing to spend to achieve certain goals afterwards. Do you want to take portraits of people or perhaps taking pictures of birds/animals or perhaps landscapes is your interest? These are all questions which you need to answer before making a decision.
The D7000 has many advantages over the D5100 in terms of settings and ISO sensitivity which is huge... remember photography is all about the light and how you utilize it. Set a money goal and see what you can get from there in terms of camera, lens and other accessories
Another huge advantage of the D7000 over the D5100 is the construction of the body and also it has a internal motor, meaning it will focus on the AF older lens, so it will give you a wider selection of lens. Also a D90 is something you should look at as it is a little older but still a really good camera.
Photography is an expensive hobby and quite frankly never ending and this is why so many people get dragged in (keeps them busy for years to come). There is always something to learn or acquire (techniques, lens, cameras, flashes, tripods, monopods and so many other accessories which will improve your pictures). Some people achieve certain lens and equipment over decades
