CentOS is binary compatible with RedHat, so it's for people who need the enterprise-ness of RedHat, but can't/won't run RedHat. An example I've heard is running RedHat in production for the support, but CentOS for devs and stating. The reason to run RedHat is usually something like needing to run Oracle or a number of other very complex software suites (the keyword is complex, not better or faster) that are only supported on specific platforms.

If you're running Oracle on Sun hardware, go for Solaris, sure, if for no other reason the support. If you're running "regular" open source software on generic hardware, just use Linux/BSD. There's nothing in Solaris that'd make Postgres faster than Linux or BSD on the same hardware.