I'm using red hat Enterprise Linux 6.4 (can't upgrade for now).
I want to use dse 5 with opscenter 6 in a new product I'm building. The environment is offline and is not connected to the Internet.
Regarding OS releases and DSE, RHEL 6.4 is not supported / tested with DSE 5.0 and OpsCenter 6, see these links for more info:
http://docs.datastax.com/en/landing_page/doc/landing_page/supportedPlatforms.html
Also product compatibility with DSE releases:
I personally prefer package installs but as you're using a configuration that might present some issues then you're probably better off with a tarball install. Tarball installs are also useful if you want to have a test platform with multiple releases.