JUnit vs TestNG

I've used both, but I have to agree with Justin Standard that you shouldn't really consider rewriting your existing tests to any new format. Regardless of the decision, it is pretty trivial to run both. TestNG strives to be much more configurable than JUnit, but in the end they both work equally well.

TestNG has a neat feature where you can mark tests as a particular group, and then easily run all tests of a specific group, or exclude tests of a particular group. Thus you can mark tests that run slowly as in the "slow" group and then ignore them when you want quick results. A suggestion from their documentation is to mark some subset as "checkin" tests which should be run whenever you check new files in. I never saw such a feature in JUnit, but then again, if you don't have it, you don't REALLY miss it.

For all its claims of high configuration, I did run into a corner case the a couple weeks ago where I couldn't do what I wanted to do... I wish I could remember what it is, but I wanted to bring it up so you know that it's not perfect.

The biggest advantage TestNG has is annotations... which JUnit added in version 4 anyways.

First I would say, don't rewrite all your tests just to suit the latest fad. Junit3 works perfectly well, and the introduction of annotations in 4 doesn't buy you very much (in my opinion). It is much more important that you guys write tests, and it sounds like you do.

Use whatever seems most natural and helps you get your work done.

I can't comment on TestNG b/c I haven't used it. But I would recommend unitils, a great wrapper for JUnit/TestNG/DBUnit/EasyMock, regardless of which route you take. (It supports all the flavors mentioned above)

About a year ago, we had the same problem. I spent sometime considering which move was better, and eventually we realized that TestNG has no 'killer features'. It's nice, and has some features JUnit 4 doesn't have, but we don't need them.
We didn't want people to feel uncomfortable writing tests while getting to know TestNG because we wanted them to keep writing a lot of tests.
Also, JUnit is pretty much the de-facto standard in the Java world. There's no decent tool that doesn't support it from the box, you can find a lot of help on the web and they added a lot of new features in the past year which shows it's alive.

We decided to stick with JUnit and never looked back.

TestNG's biggest draw cards for me include its support test groups, and more importantly - test group dependencies (marking a test as being dependent of a group causes the tests to simply skip running when the dependent group fails).

TestNG's other big draw cards for me include test parameters, data providers, annotation transformers, and more than anything - the vibrant and responsive user community.

Whilst on the surface one might not think all of TestNGs features above might not be needed, once you start to understand the flexibility bring to your tests, you'll wonder how you coped with JUnit.

(disclaimer - I've not used JUnit 4.x at all, so am unable to really comment on advances or new features there).

I wanted to share the one I encountered today. I found built-in Parameterized runner is quite crude in Junit4 as compare to TestNG (I know each framework has its strengths but still). The Junit4 annotation @parameters is restricted to one set of parameters. I encountered this problem while testing the valid and invalid behavior for functionality in same test class. So the first public, static annotated method that it finds will be used, but it may find them in any order. This causes us to write different classes unnecessarily. However TestNG provides clean way to provide different kind of data providers for each and every method. So we can test the same unit of code with valid and invalid way in same test class putting the valid/invalid data separately. I will go with TestNG.

Also one more advantage of TestNG is supporting of parallel testing. In our era of multicores it's important, i think.

I also used both frameworks. But i using hamcrest for assertations. Hamcrest allows you easily write your own assert method. So instead of

assertEquals(operation.getStatus(), Operation.Status.Active);

You can write

assertThat(operation, isActive());

That gives you opportunity to use higher level of abstraction in your tests. And this makes your tests more robust.

A couple of additions to Mike Stone's reply:

1) The most frequent thing I use TestNG's groups for is when I want to run a single test method in a test suite. I simply add this test to the group "phil" and then run this group. When I was using JUnit 3, I would comment out the entries for all methods but the one I wanted to run in the "suite" method, but then would commonly forget to uncomment them before checkin. With the groups, I no longer have this problem.

2) Depending on the complexity of the tests, migrating tests from JUnit3 to TestNG can be done somewhat automatically with sed and creating a base class to replace TestCase that static imports all of the TestNG assert methods.

I have info on my migration from JUnit to TestNG here and here.