I have some systems on SL and some others on CentOS. I haven't noticed any significant lag in CentOS updates. The only time this really became an issue was the release of 6.0 from TUV. CentOS had some problems getting it to build.
Also, the two distros are NOT identical clones of RHEL. CentOS maintains binary compatibility with TUV. SL does not not claim to be 100% binary compatible (but usually is). This may explain why there are extra packages available for SL although you'll probably find most of them in the CentOS "extras" repository or they're available through the EPEL repo.
Oh yeah, I run the latest version of Fedora on my laptop and in a few VMs. For the Laptop, I keep the latest version of Fedora on one half of a 400GB external hard drive and the other half has the previous release. This way I always have access to a stable Fedora release and just have to keep copying my local data between partitions when I install a new release onto whichever partition has the older release. Works for me and I'm always doing a new install so no fighting with FedUp to upgrade.
I've tried other distros (Ubuntu, Mint, Gentoo, SuSE, etc.). I like the stability I get with RH. BTW, I started with Red Hat Linux 5.0 in 1998.
Cheers,
Dave