My databases in Postgresql have not exceeded approximately 10GB, so for larger databases you may want additional information.

With AutoVacuum enabled, and adding "VACUUM FULL" to the end of my nightly backup routine, I have found Postgresql to be really fast. MySQL has had some performance updates since I switched, but at the time Postgresql was performing better for me. I believe MySQL was at the first release of version 5. If there is a performance difference between Microsoft SQL and Postgresql, I'm not noticing it. I have a lot of experience with Microsoft SQL, and I can tell you that prefer Postgresql.