They’re both fast enough for the majority of projects, and have a pretty similar feature set. Plus, if you’re building with a database independent framework like Ruby on Rails or Hibernate it’ll only take you a few hours to switch between them.
I’ve used both databases and I’ve come to the realisation that you they’re basically interchangeable. So don’t worry about the tech geeks arguing about which one is ‘better’ or ‘faster’, odds are they’re both fine for your application.
Having said all that, just like the FeedLounge guys I prefer PostgreSQL, and it’s the database we’ll be using for all future applications here at Joyent.