I'll just throw my two cents in:

I don't like Mandrake. Far too heavily changed from any normal standards so far as File Locations (like the FHS) and doing anything except the Point and Click with it was a pain. I prefer Mandrake's business model over Red Hat's though.. (offer 3 discs and primary OS for Free.. keep 2 "special" software discs available to "Gold"/"Silver" subscribers.)

Fedora is decent.. but, like mandrake, I got turned off it when I tried doing more advanced things. Red Hat modifies a lot of locations and uses custom config stuff that is unique to them... I like being "cross-platform" and "cross-distro" aware.

Ubuntu variations are my favorite for desktop use. You mention Xubuntu, that is Ubuntu that uses the XFCE desktop - used to be my Favorite setup because I like the Lighter-weight desktops vice Gnome or KDE.. even on Top End systems. I prefer my computer reserve it's resource to doing actual work.. and not painting pretty pictures for me.

Debian is my favorite for Server systems (short of Openbsd). No particular reason why except I'm most familiar with it.

I've bounced from Debian, Slackware, Gentoo, *BSD's, LFS, Arch, etc.. and most of those are pretty "standard" in setup and configuration files and locations.. you can move from one to the other and the most difficulty you'll have is learning the new package management system.

Anything Red Hat based (CentOS, Mandrake, SuSE) is (slightly) different.. and you'll notice it pretty quickly after dropping to the CLI and trying to modify stuff. Because of that I just use them and go "meh".

This has just been my experience.. YMMV.