Well, I did some benchmarks, and they show that, out of the box at least, MySQL is faster.
Here they are:
http://2bits.com/articles/benchmarking-postgresql-vs-mysql-performance-using-drupal-5x.html
I just found this:
Converted the site from MySQL to PostgreSQL
http://jamonation.com/node/734
it is not explicitly said but the site should run drupal.
--
Ivan Sergio Borgonovo
http://www.webthatworks.it