The differences between Red Hat and other distributions are mainly in the system administration tools, user authentication, and the bugs.
This really brings up the differences between distributions in general. Although they all use the Linux kernel source code each distribution does lots of specific tailoring. A distribution may use software tools that other distributions don't use. For example Slackware doesn't use PAM by default for user authentication. Another example is that the Ubuntu distributions are starting to use their own startup scripts and file structure and Gentoo has always had its own unique startup configuration. Some distributions may use XFree86 for their X windows software while others use X.org and still others are starting to use Weyland.
Also different distributions may use specific versions of common tools or add-ons that are not available in other distributions. These differences can be something as low level as disk drivers or may be higher level such as NFS and Samba. This means, among other things, that each distribution will have its own set of bugs. When you learn to work around specific bugs in one distribution that knowledge may or may not help you solve similar problems in another distribution.
In general, I would say that learning Red Hat is better for a professional system administrator simply because Red Hat is more popular in the business world. CentOS is a moderately good proxy for Red Hat simply because it uses Red Hat code. However, I have spent a lot of time reading the CentOS mailing list and it is clear to me that the CentOS people are fixing bugs that they find. This means that CentOS will have a different set of bugs than you will find in genuine Red Hat. The good news is that CentOS uses the same drivers, applications, and system tools so if you learn to do something in CentOS which involves using a specific system administration tool, like yum for example, then that knowledge will be helpful when you want to perform the same task on a genuine Red Hat system.
Last edited by stress_junkie; 04-29-2011 at 10:29 AM.