In this article, Noah Gift does a good job presenting three main alternatives for web app integration testing: Windmill, Selenium and Twill. Twill doesn't do Javascript, so that leaves Selenium and Windmill as your main possibilities; Gifts shows enough about what using each of them means, so you can choose.
One thing Gift doesn't mention is that Selenium is much more popular -- that's not obvious if you just web search each of the terms windmill and selenium, but that's because each gets (different numbers of;-) false hits. [windmill javascript] gives 325k hits, [selenium javascript] gives 1.2M hits, and this ratio is more representative. Anyway, the point is that, if you find them both equally easy and powerful enough for your needs, so that you have a hard time choosing one, then picking selenium (i.e., going with the crowd) may have advantages (more experts around, e.g. on SO to answer questions;-) wrt picking the somewhat less popular alternative.