Well, "My girl friend should be a b*tch, i want my wife virgin" .
If you're hosting your server in-house, you'll probably can't afford Oracle or MS SQL. In fact, you shouldn't. MySQL is no more less scalable, affordable, reliable and secure than "the others".
If you stuck between PostgreSQL and MySQL, choose MySQL for it's speed (though PostgreSQL is also speeding up), and PostgreSQL for it's native RDBMS features like triggers, procedures. MySQL requires 3rd party InnoDB storage engine, and it's acquired by someone else :)
No need to panic, MySQL immediately replaced it by Falcon on it's 5.2 releases.
I am currently using MySQL for a corporate sales project, and it's still working soooo fast after 2.5 million records. All good.