It REALLY depends on the kind of photography you want to do. But I gather that you are a beginner so I'd recommend the D5100. The thing about starting photography is that when people ask you want you want out of your camera, you have no clue what you want so it is a question you can't possibly answer. Or at least that's how I felt. So my answer is safest choice would be a DSLR. At this point, I think what I enjoy out of photography is the act of taking the pictures, and occasionally looking at them. I don't enjoy post-processing all that much. So ideally, I would want a Leica M9, or a new Fujifilm X10 (it looks so sexy and small), or what I'm actually probably gonna get the Leica Digilux 2. I like the experience of shooting more than the images. And that's kind of a counter intuitive concept when you are just starting out. The reason I always recommend a DSLR is because they are versatile. They can do it all. You can shoot sports, you can shoot nature, you can shoot portraits. And maybe the D5100 isn't the best are any of those things but it will get the job done.
But I strongly believe that a DSLR is not the end all be all to photography. Especially as a hobby. As much as I love my D7000, sometimes I find it big and intrusive and would like a smaller camera that handles better.