SoapUI vs junit

Junit is a good option to quickly have unit tests ready, simply by adding custom 'test' target and add <junit> task inside, put whatever classes you want to test, thats it. For java, or pojo's this works best.

However for WebServices soapUI is a simpler option with ui based tests that can be done very quickly. It many features including invalid payloads etc, but you cant configure your real client like situation in soapUI, e.g. Jax-ws based client. It targets to have more interoperable env to test for your service.
In any case, you can always use soapUI to quickly devise and run tests. Later you can create jaxws client from WSDL and write junit for your service.

Jai ho