I have used mysql, postgres and Oracle, so I'll speak for what I know.

mysql is fast, but... so what? With mod_perl and persistent DB connections, plus my sites don't really have that many visitors, I don't really care about the DB's speed.

Oracle is well known, has stored procedures, is capable of advanced clustering and such, but it's sooooo expensive, that I would better forget about it.

PostgresSql, is the best choice for me. It has triggers etc, it is as fast as I need it to be, it supports transactions (which, combined with triggers and rules, can take MUCH of the code out of the web, putting it into the RDBMS itself, thus being much faster than mysql IMHO :-)

The best thing about postgres though, (in conjuction with perl of course) is the ability to write stored procedures in perl (you'll have to build shared perl).

I hope I helped a little bit.