A traditional DSLR is the better choice because 1.) the built-in optical viewfinder and 2.) the better selection of native telephoto lenses.

The viewfinder allows you to track your subject more comfortably; holding a camera with a large lens is easier to do with your elbows tucked in rather than holding a camera in out in front of you. You can get an electronic viewfinder for the NEX5N, but it won't be as responsive as an optical viewfinder.

There are lots of telephoto lenses to choose from with a DSLR, from relatively inexpensive zoom lenses to wide aperture "bright" prime and zoom lenses. Not only can you affix the manufacturer's lenses, but there are also third party manufacturers lenses to choose from. While you can mount Sony DSLR lenses to the NEX5N, it requires a large and expensive adapter.