VSS is old and outdated. The database gets corrupted way too often. There's a reason why MS built TFS too.
SVN is very popular (meaning lots of community support, meaning free support), there are many tools that hook to it (CruiseControl for Continuous Integration for example) and it's rather simple to use.
You have to consider there's a learning curve if you are already using VSS and that's something that you have to weigh in your research. If the other developers haven't used SVN (or CVS) then it could be costly, although all you need is one person who really gets to know the system and then coach the rest.
We did change from VSS to SVN 4 years ago and we haven't looked back since.
