What's the difference between EC2's "linux/unix" server, and "SUSE Linux" ? Why does SUSE cost more? Is it better?
That's actually a SUSE Linux Enterprise Server (SLES) install, which is an Enterprise Linux that comes with a support contract, unlike CentOS. What you're paying extra for is the support. Why would you use it? If you want to run stuff in the cloud but need to run software only 'supported' on certain installs, SLES is usually one of them and EC2-SLES allows you to do that.