You can also ask yourself what step of learning you're at. Do you just want to build stuff? Then you can probably jump to coffeescript right away (especially with the new Source Maps feature). Do you want to get more in-depth knowledge of popular programming languages? Then stick with JS; learn how it handles types, how it handles functions, what the prototype chain is, how exceptions are thrown, etc. Maybe even read JavaScript: The Good Parts.

People build amazing sites in both languages... so you can't go wrong... it's just a matter of what you want to learn on the way. Good luck!