There's no difference between the two, when you call them with just one argument. The do exactly the same thing.
The difference between the two is only in evidence if you call with two parameters. The statements
Wscript.CreateObject("some.object", "AnotherParam")
and
CreateObject("some.object", "AnotherParam")
do completely different things:
The VBScript CreateObject function interprets the second parameter as a remote computer name and tries to create the named COM object on that remote computer; in this example, it tries to instantiate an instance of an object with ProgId of "some.object" on a remote computer named "AnotherParam". The WScript CreateObject method interprets the second parameter as a subroutine prefix to be used in handling events from the object. The two GetObject functions are similarly related.
(Adapted from TechNet,
section "Comparing VBScript CreateObject and GetObject Functions with WSH".)
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