I would recommend you testing out linux distros, setting them up either using a spare PC or vmware/virtualbox solution.
If OSX is good enough, depends on what your planning to use your *nix skills for, what systems you want to work with etc.
A nice goal is to setup your home network, with fileserver, email server and router using only standar Unix and Linux distros like Opensolaris/Solaris, ubuntu, etc.
Atleast I learn very much from just playing with the systems and testing it, seeing whats differente and setting them up to do what I want.
For Solaris atleast, a good place to start finding documentasion is:
http://docs.sun.com/
And much good info about Solaris security can be found withing the JASS documentasion:
Solaris Security Toolkit