Wether there is much difference between CentOS and Scientific Linux software wisely, I honestly can’t say. What I do know is that when I googled what the best (free) way to learn RHEL was; I found CentOS to be the most mentioned RHEL clone for this purpose. It was often said that CentOS is a 99% clone of RHEL. Scientific Linux seemed to be the second choice of many.
On the Scientific Linux forums the difference between CentOS and Scientific Linux was described as following:
CentOS is a good project with similar ideas/goals (except that Scientific Linux is a little more relaxed as far as 100% binary compatibility goes compaired to CentOS).
“Brace yourselves, we’re going in a little hot!”