I was in a similar situation; fully managed dedicated server, LAMP, CentOS. Then we decided to move to EC2. Also, I had very little systems or linux administration experience. I have almost zero experience with Ubuntu, so I really cannot speak to which is the so-called better OS.
I tried a bunch of pre-built AMI’s with minimal OS installs from Rightscale, Alestic, Scalr and Amazon. I ended up building all my own AMI’s on top of Amazon Linux, first using version 2010.11.01, now I’ve migrated all my custom AMI’s to Amazon Linux version 2011.03.01.
The decision to go with an Amazon Linux AMI vs the other AMI providers was not an easy one. I played around with and tested different setups for close to a month before I made my final decision. In the end, since I wanted to use CentOS, it basically boiled down to one thing. I figured who better to know what hardware related dependencies needed to be included in the OS than the people who designed, built and maintain EC2. Nothing against Rightscale, Scalr or Alestic.
Six months later, even though I hit a few bumps in the road, Amazon’s Linux has been quite stable. Though, I did decide to compile some of the software we use from the source (ie. php 5.3, MySQL 5.5, etc) because I ran into trouble with the pre-built packages Amazon maintained in their package repository.