I had been reading numerous articles comparing MySQL and PostgreSQL and ran across this one. I wish they had mentioned some information about the configuration of the DBMS itself, such as the size of database buffers and query optimization settings.

Also, for their MySQL test procedures, I could not find any information regarding which storage engine they were using. InnoDB is fully ACID-compliant, whereas MyISAM has reduced overhead since it does support transactions and foreign key constraints, for example. I found a good article, http://www.mysqlperformanceblog.com/2007/01/08/innodb-vs-myisam-vs-falcon-benchmarks-part-1/ , comparing the two storage engines, and the results were pretty surprising to me.

It ultimately comes down to your individual usage patterns. Using a database abstraction layer will allow you to test your application against different databases so you can see how your application performs first hand.

prior to multicore cpus asuming the scaling was achieved horizontally, MySQL has been a good choice. But today, if your systems comprising the clusters are at least dual core x2 cpu, I am lead to the conclusion, as nickb points out, that is better to use PostgreSQL.