but, more specifics:
around 6 years ago postgres got dinged for speed a lot, but i never heard anyone ding it for reliability. a lot of people liked mysql more because speed trumped reliability, and "nobody" used all those fancy postgres features (real joins, subqueries, etc.) anyway.
well, it's 6 years later, and from what i've seen postgres now outpaces mysql on all but the simplest tests. mysql is still a speed demon, and is starting to get better concurrency support, and i've heard claim that it actually does real transactions these days, but postgres is to the point that the spamassassin people recommend it as the db of choice for the bayes db.
high availability is another story. for one thing, how do you define high availability? but, postgres has various clustering options (but then, so does mysql), and failover options, and real transactions, and live backups, and etc.
i imagine oracle is still faster (not a surprise: oracle pays people to do nothing more than sit around making it faster), but if you're starting out free and don't want to have to pay out the nose when you start to scale up, you should probably be looking seriously at postgres.