I have tried Mandrake, Fedora, Suse, Debian and Ubuntu and my favorite is Suse because it is fairly easy to get started and it is also from Novell and I want to support them because they are the sponsors of the mono project but that is just my opinion I'm sure everyone has their own favorites. I also currently run Ubuntu on my laptop dual booting with XP and I like it very well too.
As for server vs desktop I think Linux is mostly a server at its core and desktop is a layer that runs on top of that aka X Windowing system, KDE desktop Gnome desktop, etc. So if you are setting up a production server you maybe don't install all the stuff you would if you were using the machine for a desktop like maybe not even install a desktop. Less attack surface from a security point of view etc.
I will say that for a Windows guy there is a lot to learn but it is worth it in my opinion. I am by no means an MS hater though I am becoming a lover of Linux the more I learn. Nothing is hidden from you on linux, you can learn as much as you want.
Anyway thats my 2 cents on it