IBM's Cloudscape Versus Microsoft's SQL Server 2005 Express: "The most readily apparent advantage of Derby over Microsoft SQL Server 2005 Express Edition lies in Derby's capabilities as a comprehensive cross-platform DBMS--not just running on Windows or Linux or Unix on the server tier or the desktop, but also running on device-oriented operating systems such as Symbian or Palm OS on the client tier. Meanwhile, Microsoft has stated repeatedly that there are no plans for SQL Server on non-Windows platforms. Obviously, if your requirements include (or might ever include) running your database apps on multiple platforms, then neither of Microsoft's SQL Server editions will let you fill that particular bill.
Derby's cross-platform abilities are due, of course, to it being a 100% Java database engine, which means it can integrate tightly with any Java-based solution in the Java stack. This also makes it especially appealing to professional Java developers working with a combination of proprietary and open-source Java application servers and development tools.
When it comes to capabilities, Cloudscape doesn't labor under any of the limitations of Express, such as number of processors. While Derby/Cloudscape doesn't use multiple threads per query, it runs concurrent queries in concurrent threads, so will take advantage of multiple CPU's for better throughput and scalability. And Derby/Cloudscape doesn't have any explicit limits on disk or memory usage, so customers won't be forced into a pricey upgrade just because their data needs cross an arbitrary line. "