Hello,
I will try today to share my little experience on using Linux, the
distribution if have tested are:

• 
  Fedora
  
• 
  CentOS
  
• 
  Ubuntu
  
• 
  Debian
  

Ok,
first we will compare them with some facts, no personal opinions,
later I will let you know my preferences.

1. Number of
   packages available
   Here
   could be some errors specially because of the lot of repositories
   each distribution may have, I will try to use only the most common
   ones.
   
   
   
   • 
     Fedora.- (Repositories enabled are: core, extras, updates, livna)
     The
     number of packages reported by yum list all command
     is: 7334
     
   • 
     CentOS.- (Repositories enabled are: kbs-CentOS-Extras, update,
     rpmforge, base, contrib, addons, extras)

The number of packages reported by yum
list all command is: 5785

The number of packages reported by apt-cache stats command
is: 24088

The number of packages reported by apt-cache stats command
is: 23851

As we can see here Debian and Debian based Ubuntu has a lot more
packages available 4 to 5 times more! This are partial numbers as
the four distributions listed here, could have more repositories
enabled, but it is just to illustrate the difference in package
available.

The release cycle is an important parameter if depending on what you
want, and what for you are going to use your Linux

We have two options with this four distributions we are studying
here.

• 
  One is the long cycle release.-
  
  
  
  • 
    Pros.: You will have long time support for your applications, and
    for all the packages, this is good if you need a lot of time to
    configure and fine tune your system, and if you do not need the
    latest applications versions.
    
  • 
    Cons.: If you need a fresh release of any package for instance
    MySQL or Apache, you will have to download and compile by
    yourself, and also take care of the dependencies, (you may use
    CentOSPlus repository or Backports for Debian), but anyway you
    will normally have to wait until you have your system up to date
    with all packages.

We have in this group:

• 
  CentOS: Which major releases are each 2 years more or less, with
  partial releases in the middle
  
• 
  Debian: Which release cycle is also 2 (or even more) years, it is
  famous for its “When it is ready” release cycle.
  
• 
  Ubuntu: Which release is every six moths but it now has LTS (Long
  term support) edition, which actually is Dapper Drake
  

We have in this group:

• 
  Fedora: Which release cycle is each six moths more or less, with
  no partial upgrades in the middle.
  
• 
  Ubuntu: Which release cycle is each six moths for its not LTS
  versions.
  

We could see that Ubuntu has some of the two worlds, it has an LTS
version for those who need stability and a fast release cycle
version for those needing the latest version of packages in it's
systems, the rest you will have to choose according to your needs.

This is another topic I want to touch, this is more abstract than
the others but I will to my best to be concise.

• 
  Fedora:
  
  
  
  • 
    Live upgrade: Not to easy not recommended, could cause problems to
    your system
    
  • 
    From the CDs: Easy to do, and the recommended option by everybody,
    normally has no problems
    

From this we can see that if you have easy access to the CDs this
is a distribution good for you, as the upgrade comes from the CDs

CentOS like Fedora is better upgraded from its installations CDs
with anaconda, so if you have easy access to the CDs once the new
version is released is a good option for you.

Debian and Ubuntu are more flexible for the upgrade than the RedHat
based ones like Fedora and CentOS.

Conclusions.-

From this small analysis I think you should think about this to make
your decision about which Distribution of Linux choose to work
(Remember there are more that 300 out there, I am only taking 4 of
them).

1. 
   If you want to have the latest software available, like Compiz/Beryl
   support the latest MySQL or PHP then you should go after Fedora or
   Ubuntu on its non LTS version.
   
2. 
   If you want to have the most tested / stable and do not care about
   the versions of your packages, but prefer stability rather than the
   latest versions, then your option is CentOS, Debian or Ubuntu LTS.
   

This apply for Servers and Desktop applications, it is up to you to
choose the one that fits your needs, it is also good to test some of
them before you go after one.

Note.- If you have a low resource machine you can look for:

• 
  Puppy Linux
  
• 
  Damn small Linux
  
• 
  Sabayon Linux
  

If your needs comes from an specific solution like

• 
  Firewall
  
  
  
  • 
    Smoothwall
    
  • 
    Ipcop
    
  
  
• 
  Disaster Recovery tools
  
  
  
  • 
    Gparted (Good for partition)
    
  • 
    Knoppix (Which is a Debian based Linux and one of the best Live
    Distro out there)
    
  
  

Update: Hi, My friend from http://polishlinux.org, has this two excelent articles that are relevant to this topic.

Please read this articles.
Compare Distros
Distro quiz

Related Links:
CentOS
Debian
Fedora
Ubuntu
---------------------------------------------------
Damn Small Linux
Puppy Linux
Sabayon Linux
Gparted
Knoppix

Also If you want to see some Screenshots

Debian Installation
------------------------------------------------------------
And Screenshots of them running
Fedora: http://shots.linuxquestions.org/?linux_distribution_sm=Fedora%20Core%206
CentOS: http://shots.linuxquestions.org/?linux_distribution_sm=CentOS%205
Ubuntu: http://shots.linuxquestions.org/?linux_distribution_sm=Ubuntu%206.10
Debian: http://shots.linuxquestions.org/?linux_distribution_sm=Debian%20Etch%204.0