I'm a big fan of simpletest because it's what I know. It has excellent support for mocking and web-testing.
But I'm always scared of stagnating so any compelling arguments to switch would be appreciated.
I don't think either is going away anytime soon. Simpletest is maintained by a small, but involved group of people.
PHPUnit seems to have a bigger userbase, which may count as an argument for switching. I'm quite happy with Simpletest though.