Answer:
There are several different flavours of Unix. Which one do you want to learn? If you know enough to know that Linux is not Unix then what aspects of Unix are you interested in that aren't implemented in Linux?
If you just want to learn generic Unix, then Linux is as good as anything else. It implements all the basic Unix system calls and runs the same programs as any real Unix. It's an ideal way for a beginner to learn the basics of Unix.
To learn a particular variant in greater detail you need to specify which variant interests you.
I've checked, and there are only four Unixes that are registered as complying to the Unix03 standard:
Mac OS X (10.5 or 10.6)
HP-UX (11i V3 Release B.11.31 or later)
IBM AIX (5L and 6)
Solaris (10 and later)
If you have the hardware, the first three are fine choices. If not, Solaris is your only option.
Also;
I think it depends on your background on how much time you´re willing to devote to learning unix. Also every person has their own individual choice and will tell you that such and such unix os is better than the other.
If you´re coming from a complete beginners background I would break it down into some steps first.
1. Learn a unix based system.
Ubuntu has been mentioned and its a pretty good start as it exposes you to a unix operating system,easy to install, is free and probably a good place to build on. Also it runs on many different types of hardware as do others but its reasonably hassle free to setup. To be honest I have never tried installing freebsd so cant comment on how easy it is to install.
Anyway once you´ve a *NIX system installed you can start to learn command line, differnt unix desktops available to you,scripting etc
From there you can probably install other linux distributions, freebsd, or solaris and decide on what you like best.
2. Unix OS with greatest performance.
What are you looking to do? Run file servers, web servers, or just everyday computer usage such internet surfing?
If its just for everyday usage then I think any of the standard linux distributions will give you exactly what you want
Once you´re unix knowledge improves you can start looking at ways to increase performance kernel re-compiling and such.
For me I was used to working with solaris at work, then wanted to try linux and installed ubuntu at home and then after a while went to work with gentoo which was tough but learnt alot about linux doing so.
