[sheff]

EnterpriseDB (who sell a commercialized version of Postgres) have a couple of MySQL vs Postgres white papers on their site, but hidden behind a registration wall : http://www.enterprisedb.com/resources-community/whitepapers-... .

Robert Haas, a Postgres committer , occasionally blogs about comparisons : http://rhaas.blogspot.com/search/label/mysql

Theres also http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL

Historically , MySQL has been more widely available on low end web hosting plans, so its what a lot of people first use when they start using databases, and a lot of web apps, such as Wordpress, support it exclusively.

Until a year or two ago, only MySQL had built in (if occasionally fragile) replication which made it popular for that reason alone. Postgres now has robust replication, with new features coming down the pipeline soon : http://www.depesz.com/2011/07/26/waiting-for-9-2-cascading-s....

I prefer Postgres, but oddly enough under Oracle theres been some interesting features added to MySQL, which is good for both.

[riffraff]

postgres is also not (yet, hopefully?) available in amazon's RDS, probably agin due to the replication topic.