<p>I'm a big fan of <a href="http://www.simpletest.org/" rel="noreferrer">simpletest</a> because it's what I know. It has excellent support for mocking and web-testing. </p>

<p>But I'm always scared of stagnating so any compelling arguments to switch would be appreciated.</p>

I haven't used SimpleTest myself, so I can't say much in a way of comparison. Just by observation though, the PHPUnit syntax seems much more verbose.
The
PHPUnit Manual
is a great source of documentation, and covers most areas of the framework. My only complaint about the manual is that some areas lack detail.
My main reason for using PHPUnit over SimpleTest is that it has great
Phing
integration.