Mysql has great replication support which everyone who does webapps requires. Postgres has bolted on solutions which all don't work well for different reasons. It's really good at all the things that webapps don't tend to use when they get big though. Postgres is far the better database in terms of correctness and sometimes even in terms of speed, it's just that the web use case doesn't need real databases but replicated (fairly dumb) data stores.