Actually, both products are great and do have their strentgh!
At the end of the day it's about the details and we're seeing a tedency within direct client comparismanets.
Have a look into the following intro:
WebOffice – an easy to administer WEB GIS application framework for ArcGIS Server to publish enterprise data via the intranet or internet including multi-language support, user management (SSO), customized queries, editing, printing and reporting, etc.
This product brings the power of a desktop GIS product into your web Browser. It’s an open and extendable architecture which is fully controlled by the customer by having an administration tool for customization available (configuration instead of programming). - http://weboffice.en.mysynergis.com
WebOffice videos: http://weboffice.en.mysynergis.com/video/