In my opinion, it all depends on what you are more interested. Linux is all the rave right now, but to me it seems that it's more to do with the developing GUI environments that are being included in them that make them more user friendly.
The basis on the the networking feature of Linux is being left behind as the desktop features are becomming more prevelent.
I would never tell anyone to not install Linux (especially on a PowerPC based machine) however, you can also go to
http://developer.apple.com and download the binaries of Darwin and run a terminal based system that you can learn the guts of the the system and how all the different parts of it work.
Its a lot more work as there is no GUI to turn to (outside of X11), but I believe you can now use to Fink to get a running version of the KDE environement running on Darwin.
It all depends on what you want to do in the way of server. Both Darwin/Mac OS X and Linux are able to natively serve up the same protocols, both ship with the same (or very damned close) version of apache Web Server, Linux ships with sendmail, Darwin ships with sendmail, a lot of the utilities for the CLI are the same on both systems.
The only thing I can say about Linux is that it seems to run better on an older machine where Mac OS X requires a lot more system resources and runs slower.
The Darwin distros however are fairly CPU tolerant and run quite well on older machines. The only differences between Darwin and Linux that I can really think of is that Darwin serves up netinfo databases natively whereas Linux serves up LDAP services.
Your choice, get them both, try them both and see which one you like more. There is defineately more help to get and turn to when running Linux as Darwin is a much smaller community. Hey, they are both free so how can you really go wrong with experimenting???
