Originally posted by: Nothinman
I would suggest not worrying about the certs unless it's a job requirement or something. Concentrate on actually learning the system, anyone can pass a test.
Agreed!
If all you want is... general experience... here's what I recommend (and do):
I've been running web sites for years (since '96 and multi-line dialup BBSs prior to the web) - owner/admin/mod - all over the world - BSD, Linux, IIS. They all have their pro n' cons, but you asked about the Big-L...
Running a Linux-powered web site - mail server - writing code - creating and maintaining sql dbs - and so forth, and so on, is an excellent way to get a handle on maintaining server apps. My current server distro of choice is CentOS - so add that to your list. It's very popular, and might afford you a job at a noc, tech support (or whatever), if you become proficient.
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EDIT - Oops! I just realized CentOS has already been recommended twice. Good choice!

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The thing is, servers are only half the battle. For the most part, you'll be shelling into a remote machine and working in /home, depending on your creds. Unless you're independently wealthy and are colo'ed, run a dedicated server (managed or unmanaged), et cetera, you'll never be able to get down to brass tacks. VPS is a good alternative, but the site will be running in a VM, and it won't give you the full experience, e.g. hardware indoctrination.
Desktops are a whole different ballgame. You can have all the experience in the world running servers, trying to keep your RAID arrays intact, et cetera and not know jack about how to properly implement Linux on the desktop, configure a high-end video card to work with buggy Flash software, compile drivers for weido WiFi cards, setup your CUPS, and all things specific to desktop boxes and notebooks...
Ubuntu is okay, I guess. I run it on some of my desktop machines - but, I prefer Linux Mint on my portables. Why? Mint is drop-dead gorgeous! The artwork is second to none, and it gives you bragging rights when showing off your laptop/netbook. Ubuntu is hideously ugly, and an embarrassment. I reserve it for machines that nobody will ever see.
Aesthetics aside, I wouldn't recommend either one as 'learning tools' (in the narrow sense). Ubuntu purposely makes certain things harder than necessary, so grandpa (or whomever) can't mess it up - and ridiculously easy in other areas (so any idiot can use it). IMHO, you're not going to learn much from Ubuntu. Linux Mint is a polished turd -- it's Ubuntu with lipstick. Mint makes up for some of the inadequacies in Ubuntu (and it's a joy to use) but not sufficiently problematic to exercise the brain.
Really, I would suggest installing (and uninstalling) multiple distros. That's what I do. It's a hobby of sorts. After a while, you'll start seeing where they cross-over... and that's where the general knowledge will come from, e.g. knowing how to do anything in any distro -- not just becoming experienced with Ubuntu, but not knowing grunt about OpenSuSE or Fedora. Installing a distro and sticking with it (exclusively) through thick n' thin is the way petty biases are formed. It becomes the ol' Chevy vs Ford argument (or Pepsi vs Coke) except with distros, you know?
I recommend going to DistroWatch and downloading (and installing) the Top-10 distros. Spend about a week with each one, and in a few months you'll have a good idea about how Linux on the desktop works.
I prefer a 12-month data spam:
http://distrowatch.com/index.php?dataspan=52
Finally, once you're a know-it-all... branch out into the fringe distros. Linux Puppy, Gentoo, FreeBSD, Solaris. They all have their place(s) too, but they aren't mainstream enough to waste a lot of time on. Sure, they're interesting (and favored for some specific purposes) but they will serve to distract you from your stated goal (as I understand it).
Have fun!