Look at the features that are offered by each database and figure out which ones you need.
MySQL - traditionally choosen for simple databases that need fast SELECTS, with minimal UPDATES/INSERTS. Many news type sites use MySQL.
PostgreSQL - choosen over MySQL for complex database applications that need more power in the database than MySQL provides. Items like Foreign Keys, SubQueries, Triggers, Stored Procedures are the benefits on PostgreSQL that don't exist on MySQL.
Now I haven't researched MySQL in awhile so things may have changed, but there original mandate was to build the fastest SELECT database and were leaving out alot of the extras that are needed for a solid database application.
