I think there's even a third one, TestGen4Web, and possibly more.
While simpletest is great, it is much harder to use. It would be very convenient if we could settle for one of the browser based thingies and tell people to mail their tests. Everybody can use a browser but not everybody can write simpletest tests. So I see these as useful complements, not replacements.
I just tried CoScripter, and it's very easy. Here is a sample script to test Drupal 6 book feature: http://services.alphaworks.ibm.com/coscripter/browse/script/1369 Notice that what you read on this page *is* the actual test, nothing else involved. You can run it on your own local Drupal install. The sentences containing "you" mean that human intervention is required. There is a problem currently with this test, it fails to select the right option in the book drop down menu. Still a neat idea imho. Philippe