[artificer]

Since one can easily find comparisons, benchmarks, gotchas etc ( you can always start here: http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL ) the one thing that made me decide in favor of PostgreSQL is this (quoting a blogger):

MySQL is an open-source PRODUCT (and can be bought as such). Postgres is an open-source PROJECT (and can't be bought, and will be around as long as there is a sizable developer pool).

http://www.jiaozhoujob.com/newsprint-995.html

EDIT: PostgreSQL also seems to take support very seriously. Their latest security advisory has a patch back to version 7.4, which was released in November 2003.