I personally much prefer Nessus..
It has a better feel and management, not to mention the updates offered..
Furthermore the control of nessus via updates and usage i believe is more professional because of the proprietary model. its just easier to use
OpenVAS server is a forked development of Nessus 2.2.
The fork happened because the major development (Nessus 3) changed to a
proprietary license model and the development of Nessus 2.2.x is practically
closed for third party contributors. OpenVAS continues as Free Software under
the GNU General Public License with a transparent and open development style.
Although OpenVAS was forked, since then 2008, OpenVAS has changed into something new
with new features and functions not offered in Nessus..
For a simple desktop version assesment (1 user - small amounts of checking) - i would go with Nessus
However because OpenVAS is an open source product, people are saying its scanning abilities are a little further along than nessus.. ( i cant prove this, nor do i really believe it :P )
In a nutshell choose...
quicker updates -> good scanning = Nessus
slower updates -> better scanning = OpenVAS
Hope this helps :D