Selenium and HTMLUnit are somewhat similar in concept, but Selenium is more mature/robust and has a lot more features.
Note that Selenium encompasses the recording (IDE) plugin for Firefox, which allows you to record tests and the RC/WebDriver automation framework which essentially drives a browser. The two can be used together to make test creation very easy.
The only advantage I could see to using HTMLUnit is that it is less resource intensive, so you could potentially run tests on less hardware, but with Selenium's parallel support even that isn't really true anymore.
