I was confused about why my new server had CentOS 5.9 installed on it, when CentOS 6.3 is available.
But CentOS 5.9, it turns out, is actually the latest release.
Why are 5.x versions being released when they are a dead end that will lose full support barely 1 year from now?
Is there any reason to install CentOS 5.9 on a new server instead of a 6.x? Is there any reason to avoid 6.x? I assumed that 6 > 5.
point releases update the main branch, so 5.1 is newer than 5.0. When 6.0 came out it got its own point releases (6.1, 6.2, 6.3). These are usually bug fixes, security fixes, or minor updates to packages - there's no major re-engineering that happens in a point release, but you shouldn't ignore them if one comes out for your version as it could be patching security holes.
hence; 5.9 is the newest 5.X release. 6.3 is the newest 6.X release.
The only real reason to install 5.X over 6.X is if you have to. i.e: some software doesn't yet work with 6, which is usually unlikely but depends on the software in question. Also many people wait for the first point release (6.1 say) before leaving the older branch - to allow time for any slightly more serious security issues to be addressed, should any occur.