Junit Cons :
1. Each Test object is instantiated over and over again for every Test method.
2. Need to use statics to maintain any sort of state across invocations.
3. Relies on a static programming model which makes it difficult to run a subset of tests.
4. Have to start all your methods with 'test' and extend a base class.
TestNG pros:
1. Runtime configuration is in a XML file rather than a java source and so you can specify test groups and other relationships between tests. This allows useful stuff like being able to have a specified test run order, dependent tests (test X requires Y and Z to be run first). So for example, if you have some kind of 'verify db connection' as the first method, you can specify that all other tests depend on it, so once the first one fails, the others are not run since we've had an early failure.
2. The grouping support is quite powerful, since you can arbitrarily slice and dice your test into whatever groupings make sense for you. So for example, you can specify that a test is part of the 'functional' group, the 'config' group, and the 'database' group. You are then able to execute all tests within a particular group.
3. It extensively uses annotations (or JDK 1.4 javadoc comments) for metadata. The annotations available are a pretty rich set of controls to customize and specify test behavior. You can for example specify a timeout on a per test basis (if this takes more than 10 seconds, warn etc.)
Test NG Cons:
Tool adoption. For example with WSAD I can point at unit test class and use debug/run as a Junit program etc. Also there is this this JUnit EE that is a web application to test EJB modules that run inside the J2EE container.
