Merging Authorities: In Horizon, you can pull up the MARC records of duplicate authorities and then merge them in real time. You cannot do this in the current iteration of Symphony. I have been told that there are utilities to do authority merges though.
Views: In Horizon you can modify any view, so that you could make the patron registration screen data display in an order similar to your registration form. Or change the order of any screen to make it easier to use.
This cannot be done in Symphony workflows. Although they have said you can modify the web client pages, and using the API could create your own apps with the looks you like.
Any other big differences? or have you heard different about these two differences?