Comment Needs assessment? (Score 2) 319
How much time do you have to invest in this project, and how deep does their knowledge need to be?
I learned more from doing a slackware install (back in about '98 or so) then from all my experience with other Linux installs. I've heard people say similar things about Gentoo/Portage, so YMMV, but a distro that more or less forces people to do things by hand will both teach them, and teach them respect for, the system. You mention two systems that use apt, and one that uses rpm... Pick one architecture, your IT staff will thank you later.
You may simply want to give them an up-to-date Ubuntu (or Mint) that has several window manager/desktop environments installed, and let them experience the different UI flavors available... assuming that your company hasn't made the decision already. As someone else not-so-shallowly pointed out, you should have made a decision already, so train them on the distro your core dev team is using! Seriously, there are major support implications of allowing joe user to run off the flavor-of-the-month they just downloaded on a whim from distrowatch...