The Utah PHP Users Group site has this new posting from one of their members taking a look and comparing PostgreSQL and MySQL.

Recently a very good discussion took place on the UPHPU mailing list regarding MySQL and PostgreSQL (a.k.a. pg or pgsql), the two most popular open-source database engines, and when you might want to choose one or the other for your project. Many great insights were shared, and we're sharing some of them here, in hopes that it will help others in the future.

Probably the biggest and most frequently discussed differences between MySQL and PG are in features and complexity. [...] Another aspect to consider is support. (Also) many people see performance (speed) and capacity (both volume of data and volume of queries) as key factors in choosing their database.

He seems to lean a bit more towards PostgreSQL, but does make it clear that both have their places in the online community...