With SOAP UI the support for WSDL is obviously better but I must admit that groove or the other languages supported by SOAP UI are not so easy to use for Authentication.

I actually wrote a lot of web service code in Groovy for many years, so I have a bit of experience with it. Does SoapUI allow you to use or add third party Java or Groovy libraries? If so, you can use ScribeJava with some custom classes for easy TBA authentication, or the Google OAuth Client Library for Java which fully supports NetSuite’s TBA from version 1.32.1 and up. Using either of these libraries will make it much, much easier than trying to implement TBA authentication yourself.