It seems there is very few comparison between Selenium / WatiN and SimpleTest (which has web testing features too).
I tried Selenium and found the GUI great to create tests as you can see what's going on and record without typing all commands manually.
As for running the tests, Selenium is way more complex than SimpleTest. For SimpleTest you just have to run a PHP script which does all the tests (client or browser side). This means that whatever browser or browser settings you're using, you can simply go to the test URL and it'll work just the same.
What would be useful here are some comments from people that used Selenium or WatiN: Why is Selenium so famous? In other words, what would be the main benefit of using Selenium for example, over SimpleTest?
PS: Please exclude reasons like "SimpleTest requires PHP"; that's pretty obvious is PHP is included in most LAMP anyway.
Since SimpleTest only deals with the HTML content of a page you can't test pages with it that rely on JavaScript behavior. At the end of the day it's a speed and functionality tradeoff.
BTW, Selenium can be integrated into a PHPUnit test suite: http://www.phpunit.de/manual/3.1/en/selenium.html