Overall, the decision to go with SQL or SQL Express should be made according to the performance limitations inherent with SQL Express (there is a limit to only 1 CPU and 1 GB of RAM for SQL Server 2008 R2 Express and 2012 Express) as well as to the overall amount of metadata, folders, security, etc. (all of which are stored in the database, which is limited to 10 GB for 2008 R2 Express and 2012 Express) that the Repository will contain.

Obviously, every system is different, but in general we’d say that SQL Express would be sufficient for a system consisting of 10 to 15 users performing mostly browse and read type activities. If a system contains more users or requires more frequent performance-intensive tasks (such as searching, importing, etc.), then SQL Express may not be up to the task. The nice thing about SQL Express is that you could start out with it and then upgrade to full SQL once your system has grown to the point where it is the limiting factor.