In my 'limited' knowledge of the two, I do however have both.
I used QTP for testing mainly GUI interface, WebUI type applications.
I have not used QTP for testing WebServices especially if the code is using SOAP::
SOAPUI - for the most part is specific to Web Service testing.
I narrowed it down that the Open Source version does a lot from simply SOAP response, to "Contains" Responses and Performance testing from your WSDL files.
Your question made me open up QTP to look around and I searched on WSDL and it had limited information. I Searched for SOAP and found nothing.
I think HP is comprehensive to figure out what to do about Web Services Regarding SOAP Service. Rather gives me a headache looking at QTP.
SOAP UI though is so simple... I pull the WSDL file and I'm pretty much ready to go... Just add some Assertions and things and that's that.
Rob