They each have their loyal followers for good reason. They are stable, fast and free.
They are different.
- installs easier
- upgrades from version to version easier
- hides the "root" account from the user
- has a more useful "run live" CD
- is more tailored to Red Hat Enterprise users
- can require more fine grained security understanding
- is often a bit more "bleeding edge"
- in my experience does not upgrade from version to version well
- requires a root password be created at install
I personally do not use Ubuntu.
I hand out Ubuntu "run live" CD's like candy.
Ubuntu installs easily enough that a novice user can successfully put it on an old machine and have it running in minutes. I recommend and install Ubuntu for friends and family.
The Ubuntu "run live" CD's are also great for recovering the data from crippled Windows boxes.
So, there you have it. I run Fedora and recommend Ubuntu.