Both are same if all the elements used in the
soap message comes under the same targetNameSpace. (ie. if 'http://schemas.xmlsoap.org/soap/envelope/' is your default namespace).
But in webservices it is a common practice to define the complex elements in an XSD. While creating the SOAP message, these elements should be have a prefix that refers to the targetNameSpace of that XSD.
For eg.
if the XSD that defines the complex element 'Employee' has a targetNameSpace "http://www.sometns.com/", SOAP message looks as given below.
Unless we prefix Employee element, the receiver will not be able to understand the 'Employee' element. Having prefix, receiver can relate it to the corresponding XSD and identify the element.