It depends on exactly what you are trying to learn.

CentOS is a very good Linux distribution. Linux is generally open source and support by a vast user community.

Solaris on the other hand is Unix, owned by Oracle and is a big boys operating system, commercially supported, and well capable of running a big financial institution such as a bank. You wouldn't run an operating system in a bank without full commercial support services (unless you are mad).

Learning Solaris, HP-UX, AIX and the like is certainly professional stuff which cannot do you any harm at all.