Generally, i choose the OS depending on which application i am deploying, and what the application is built for. Older applications, or configs are available for older OS , which are complete with a lot of information. Newer OS may not have packages for them. If it is a web server, or database, centos 7 is fine
What are you going to do with this OS?
If you are looking to use it for shared hosting, with cPanel/DirectAdmin/Other, I would recommend CloudLinux which is CentOS compatible.
Either way, go with the v7 variant, the kernel is newer and you are currently unaware what CentOS 6 will be incompatible with in the fuiture.
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