Oracle is much versatile than MySQL. Oracle can run and handle more transactions than MysQL. Oracle would be better if you are going to have lots of db usage.
I've used MySQL. Only for a forum. Still, MySQL takes up resources with PHP that affected the server load. Oracle on the other hand is better with memory leaks and handles multiple threads. You can do SQL Transactions, stored procedures, DTS in Oracle that you can not do in MySQL earlier versions.
Oracle is expensive, although they came out with Oracle Free Edition to attract MySQL users.
MySQL users enjoy this database because it is available on virtually all hosting companies. Oracle are only limited to large corporations due to licencing costs and not all hosting companies have Oracle (they might have SQL Server, but not Oracle).
You can make billions of data with MySQL, but it will be slow on the server. In addition, backing up the MySQL database takes forever, versus Oracle's tools which makes database backup a cinch.
If you are going to use MySQL, then add lots of memory. In addition, I recommend that you seperate the Website from the database. In other words, have emails and website on a seperate server and only have a dedicated MySQL server specific to mysql/db only.
MySQL is Free, It's everwhere, that's the whole concept!!!