The best comparison for WSUS Offline Update would be the Microsoft Baseline Security Advisor (MBSA). It can be used in an offline mode, and then it uses the same file wsusscn2.cab as WSUS Offline Update. This file can be found in the directory wsusoffline\client\wsus.
MBSA will only search for important security updates, since this is what the file wsusscn2.cab provides. It may find a few updates, which are explicitly skipped by WSUS Offline Update, like the "Malicious Software Removal Tool". This is a rather simple anti-virus tool.
The Microsoft Update web site will always find more optional updates, but there should be no missing important security updates.
So I wonder, how this comparison turns out for Windows 7, 64-bit. If the MBSA doesn't find any security updates, but Microsoft Update does, then Windows 7, 64-bit would be broken.
http://www.microsoft.com/en-us/download ... px?id=7558http://www.microsoft.com/de-de/download ... px?id=7558Edit: corrected Basic to Baseline