IPv6 support for Disaster Recovery

This really depends on your DB size and your comfort zone.

SQL Server Express edition is a fully functional free database up to 4GB in size; perfectly adequate for most small to mid-sized apps. (aside - IMHO, MS Licensing was the best thing ever to happen to the open source community!!)

PostgreSQL is a fully functional, free database w/o the 4Gb limitation - however, PostgreSQL follows the Oracle style of SQL and database management. If you're not familiar with that, you can run into some gotchas.

Throwing Dot Net into the mix adds a not insignificant wrinkle. Can you use .Net with PostgreSQL? Yes, but Dot Net will work better with SQL Server. Are you comfortable writing complex ODBC interface code? If yes, PostgreSQL remains a contender. If not, you'll be better off with SQL Server.