I won't be able to address a big list of pros and cons of the distros
you listed.
Instead, I will pass along this... you have the choice to try any, (and all) of
the Linux distributions you have listed. I think the best way to know for
yourself the pros and cons, is to try each of them out.
Now - I know people will say "HOLD ON, Red Hat Enterprise is a commercial
product- you CAN'T just use it for free!!" and they'd be right, but you can use
their derivitives Fedora or CentOS.
1. Fedora, and CentOS can give you a similar experience to Red Hat [ as their
code is based upon Red Hat] and both are free. There is no commercially-based
application support - but don't worry -- the communities (via Forums,etc.) are
very helpful.
2. The other distros you mentioned (Ubuntu, Kubuntu) are based off of Debian.
What separates them in the obvious sense is how they behave. There are many
other pros/cons to both.
For example, in 'debian' derived Distros, you have to put in a sudo password
(namely, your user password) for many tasks such as installing software or
updating
the installed packages - along with other commands.
In Red-hat derived distros, you don't get prompted as much for your own
password.
In any of the distros you mentioned, you can mess around with different
windowing environments and file managers.
Lastly, you can try out so many distros by burning LIVE CD versions of them.
Use them, see which ones you like best, and go from there. It's all about
choice.