Debian has 3 versions:
Stable - packages 6-18 months old. Numerous patches and security patches applied. Stable but crusty! Good for server installs
Testing - packages 3-6 months old. Pre-stable 'staging' version
Unstable - latest and greatest packages. Whoohooo!
This site is good for showing what the current packages are:
http://www.distrowatch.com - also good for news and all distro information
My advice is install stable then switch to unstable. Change all references of 'stable' to 'unstable' in the sources.list file and then simply 'apt-get update' and 'apt-get dist upgrade'. I run unstable and rarely have had any issues. I upgrade all my packages about once a month to keep everything fresh.
I too started on RedHat and then tried Mandrake, Slackware, Gentoo and Debian. My thoughts:
Mandrake - good noobie distro, easy install, very much like Red Hat. RPM based, ugh!
Slackware - very stable distro. Upgrading can be a bear!
Gentoo - very promising but not stable. I clobbered my init scripts when doing package upgrades. Install takes forever even with broadband!
Debian - awesome package system! Install once and upgrade away! The package scripts even make menu entries for you. Slackware users can appreciate this. Stable for servers/unstable for desktops
I would of stayed with Slackware if it weren't for the package system. Botom line is, know your hardware and what drivers are needed. Good luck!