mojoPortal supports also MySql and SQL CE (SQL Compact Edition), SQL CE is the easiest to install but the least robust for performance and load. MySql is pretty robust, this site uses it, but it requires extra installation steps to upload the mysql data layer package for mojoPortal. So if MySql is available at no extra charge you could use that, or you could use SQL CE but I would not use SQL CE for any real important site where you want to have high traffic. Also our WebStore feature is only supported for MS SQL or MySQL so if that is important to you then it rules out SQL CE.
SQL Server is the most robust db platform and generally what I would use in most cases.
Note that our codeplex download page has different packages to install mojoPortal for different db platforms.
Hope that helps,