We use these two databases to keep it simple and consistent. The web engineers know there databases well. We have over 100 web sites to keep running.
I generally use one these days, MSSQL. I’ve found over the years that what’s far more important for performance than the database engine or even the hardware you use is the design of the database and how it’s used by the application. I’ve achieved literally 100x+ performance improvements combined with 90% drops in server load by changing even small parts in the structure and access of databases created by others. They thought SQL Server or the hardware was just slow, but it was the design that was at fault. Why throw $50,000 more at hardware and SQL licenses when a few days of intelligent analysis and redesign will fix it?