Emacs -- provides a functional and highly customizable editor. It's got a lot of very nice packages (org and magit, for example, are both superb). It also has a different user interface paradigm -- it's usable entirely from the keyboard. Once you are used to this moving back to something with all that clicking around is rather hard to cope with. And it's very easy to add new functionality.
VIM -- like Emacs, it is entirely usable from the keyboard. It's not as functional as Emacs, but is it very regular. The main editing commands are very predictable which makes the raw editor of text very efficient.
That's about the best quick description I can give -- I am mostly an Emacs user, and use VIM for systems administration, so there is a bias in what I say. They are both fantastic tools and it's worth trying them out.