task.js generators/promises vs. async callbacks

I want to know which of the two methods are better when dealing with asynchronous code in JavaScript. I want to understand which method leads to cleaner code. I'm used with promises and they seem more flexible than the async approach (https://github.com/caolan/async).

I also know about the Task.js library (http://taskjs.org/), but this relies on the yield keyword which is part of Ecmascript Harmony.