Learn Javascript. Coffeescript is defined in terms of Javascript. If you don't know Javascript, learning Coffeescript will be unnecessarily difficult, because you'll have to go back and learn Javascript in bits and pieces as you learn Coffeescript. If you learn Javascript first, learning Coffeescript will be easier, because Coffeescript will build upon your Javascript knowledge.
To re-emphasize this point, there is no "choice" involved. You cannot understand Coffeescript without understanding Javascript... it almost looks like you can at first, but at soon as you start debugging you're back in "what is this prototype thing?"
On the other hand, after you work with Javascript for a little while you quickly start wishing for something like Coffeescript. It's a very natural transition, but you can't short-circuit the process.
