The optimizer is smarter and it's better at concurrency control. You'll get better performance out of Oracle when you have very-long-running transactions or lots of transactions at once operating on the same tables. The former isn't a concern for most web applications, but the latter might be.
The above is received wisdom from academics, though, so it may be dated. PostgreSQL has improved a lot in recent years.
We chose Postgres over MySQL by a very fine margin, but we're pleased we did. Seems to have fewer "try to make your life easier but end up doing the opposite" quirks.