I've used Progress on CentOS, Red Hat, and SLES. They have minor functional differences in terms of package versions, software installation and updating, system setup, installation defaults, etc.
In practice these differences can be far overshadowed by the quality the sysadmin(s) who will install, configure, and maintain the OS. If you have a capable SA, any of these distros will serve as a good DB platform. If the SA is the type that accepts all of the OS installation defaults without question, doesn't do proper maintenance, says "Progress? Not my job to know about that", or just generally doesn't know what they're doing, then you're in trouble.
As far as OS versions are concerned, follow CJ's advice and check the v11 guide. It's updated several times per year.