I believe that there are quite a few differences. Whilst from the looks of it Nakisa did help SAP develop Workforce Viewer it is far from being as feature capable a product as SOVN OrgChart and it provides no other functionality outside of some basic structure visualisation.
I'd recommend taking a look at the following for more information.
- HR Renewal 1.0 Feature Pack 4: Why did SAP introduce an own Workforce Viewer?
- Workforce Viewer - SAP Library
- HR Renewal 1.0 Feature Pack 4: New Org Chart functionality – The Workforce Viewer
At the end of the day, if you just need simple charting then Workforce Viewer will likely meet your needs. If you need more flexibility (e.g. to customise what is shown/not shown) or capability (exporting, analytics, etc.) then you need to upscale to SOVN OrgChart or an equivalent product.
Hope that helps.
Regards,
Stephen.