Ask yourself these questions:
Who is going to support your website (and database)? There are enterprise db companies out there which can provide world wide support, documentation, consultants etc. It is not just Oracle, but it is very likely that it's easier to find Oracle or MSSQL specialist in every country.
Now, if you are willing to take a challenge to find PostgreSQL specialists or you are willing to train your team for PostgreSQL, go for it! Today (2016) PostgreSQL is the most advanced open-source RDBM and is on-par for most deployments with commercial competition.
Are there required features that only one database system offers? You cannot use advanced features of both database platforms, so check what it is that your application really needs. For example, if you require specific replication, backup and other scenarios, you should read documentation for both platforms to make an educated choice. They both offer similar features, but its better to be prepared.
Big companies usually stick with one database platform. When they buy an Oracle license once (and employ DBAs and other specialists), they continue using that vendor. That's the usual scenario, but I assume you're free from such baggage.
Also in the modern IT era, another question is to consider. Do I really need RDBMS? Every insert/update/delete costs very much if you compare it to let’s say appending to a plain file. When you define foreign keys and other constraints the cost is even higher. Explore other solutions or even plain data files, RDBMS might not be the ideal solution. Options like MongoDB, Redis, Memcache, plain files, dbm or object databases. There are many options.
Beware that usually you want RDBM in 9 out of 10 cases tho!
ps - I worked on a massive MongoDB project which turned into a disaster and few years long refactoring. Tho it might be a valid option for SOME use c
answered 2011-05-12T09:36:55.160