If you don't have any specific requirement and just need to run your website, CentOS 6 is preferred. Install CentOS 6.3 and run "yum update" after completing installation; it will update the system to the latest version.
CentOS would be the best choice. CentOS 5 is a good product, but it will hit EOL well before 6.x will so to keep from having to deal with any upgrade hassles for a long period 6.x would be the best choice for a long term commitment.
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