SimpleTest is slightly easyereasier to grasp , but PHPUnit is the best ( in my opinion atleastat least ) , so if you whantwant to start learning and using a framework , start with the one you're going to use when you'll be a master in TDD , don't. Don't look at if it's easyereasier or harder now , becousebecause if you start with SimpleTest for example , then you start using Zend Framework becouse you'rebecause your boss tells you so , you'll have to use PHPUnit with ZF . So think of you'reyour future , becousebecause an easy to learn framework doesn't necesarly hasnecessarily have to be the best one , usualy the harder to grasp the better the framework .
AllsoAlso have a look at they'rtheir revisions , see witchwhich one is maintained better .