Check out the Teabag project on github. It specifically allows you to use the asset pipeline along with Mocha, Jasmine, or QUnit.
https://github.com/modeset/teaspoon
Mocha's a pretty great library, but I typically use Jasmine -- for me it's primarily about knowledge and experience with Jasmine, but Mocha has some really cool features -- like letting you know when you're bleeding things into the global scope.
Figured it was worth mentioning the project because it specifically allows you to play around with both (in different suites) so you can decide for yourself.
