Woah, don't need to download both by any means. Seriously, they are the same distribution with a different desktop environment.
If you have an ubuntu cd you can install "kubuntu" by :
Code:
sudo aptitude install kubuntu-desktop
and vice versa, you then get to play with the ability to have either desktop available to you (I personally use xfce, so in ubuntu the easiest way to get xfce is xubuntu-desktop)
Ubuntu is a very good start out distro, it can do a lot of powerful things but is also generally not too complex.
I also run debian (which ubuntu is based on) for my server, since it is exceedingly stable and pretty much bulletproof.
What particular things do you want to do with your linux, as it is not necessarily the tool you need, but definitely worth a look.