knoppix is sweet for the bleeding-edge workstation, but it is NOT a very good choice for a server (we ARE talking about servers, right?)... you need something more "serious" and "proven" on a server...
debian linux is used on mission-critical systems by LOTS of companies and governments...
you can get a glimpse here (along with text descriptions):
http://www.debian.org/users/
(i say "glimpse" because a lot of folks simply don't say they are using linux, so they can keep one-step ahead of the competition that's still paying for those crazy licenses from "you-know-who")
debian has PLENTY of advantages over other distros...
but before i get flamed, i'd like to make it clear that there is no "best" distro... it all depends on what you want and what you need and what your taste is and what your budget is etc...
having said that, what most people appreciate about debian is how obsessed the debian development community is with STABILITY AND SECURITY... those are the main driving factors behind debian... other distros give priority to other factors such as "ease of use" and "marketability" (does that word exist?) and "bleeding-edge" and "coolest window manager" etc...
another thing is the fact that debian is very strict as to the licencing of everything in the system... all the software included in debian is thoroughly checked to make sure it is 100% free software... non-free software is left as an OPTION... and for those of you that are inside the united states, debian goes one step further: it complies thoroughly with the united states's tight regulations on certain software... you just let the installer know you are in the states and it won't install any customs-regulated software... this is way cool (a lot of folks see this as a negative, but i feel the law is the law)...
so if you absolutely cannot have any doubts as to the security, stability, and legality of your software (such as is the case when you work in the government), debian is IMHO the way to go...
i'd also like to point out that unlike mandrake and the like, debian IS NOT A PRODUCT... it is a PROJECT... it's a PUBLIC TECHNOLOGY... it's been around for MORE THAN A DECADE... there is no "company" that will go broke... you can rest assured that tomorrow debian will still be there even when the others go under...
on a finishing note, you mentioned that your background is in UNIX... i must tell you then that the most UNIX-like distro is slackware... you might find it very comfortable if you are very used to UNIX:
http://www.slackware.com/
http://www.slackware-advocacy.org/
GOOD LUCK!!!