Amazon EC2 Ubuntu or CentOS

Hi,

I am switching over to Amazon EC2 from my current shared hosting provider. I am planning to use Virtualmin, but having some doubts regarding the Operating Systems. Should I go with Ubuntu or CentOS. I am very much familiar with both distros and can work on both OS. But worked on Ubuntu Server longer than CentOS.

However, I am not sure which OS will be better based on security for long time. I am not very good from the point of security. I am also planning to launch an app based on the website contents soon, so there will be lots of load and security concern. The problem if I start with any of distro, I will not be able to switch back to other distro very easily, without spending days and affecting my site and app.