I think your answer is also going to depend on how much you're willing to use the full powers of the database.
If you're really using the database to its fullest abilities, Oracle is going to be a winner for you hands down.
If you're looking for easy maintainability, Postgres will win hands down.
If you're competing on price, well, do I even have to mention which will win?
Oracle has some powerful functionality that is unbeatable in certain circumstances. With 11g, you'll now have the ability to pretty much always keep your database up. You'll be able to upgrade pl/sql pacakges without downtime or invalid db objects.
And the performance of Oracle on large large databases is pretty hard to beat, especially when you start talking about analytic functions. Oracle in general latches much less than other databases, which gives it much more scalability. But most sites really won't run into these issues unless they're huge or really pushing the envelope.
Oracle is definitely more mature than Postgres, but Postgres is improving quickly. It's no longer as easy a choice as it used to be.
If you want to understand where Oracle shines, read any book by Tom Kyte.
