SME Server - whatzat?
I'm a programmer and have done a lot of work on Linux/Unix. I have to honestly say that I've never heard of SME Server until today. It's a fork of a VERY old version of Centos, which is the open-source version of RedHat Enterprise. (Because of an agreement, Centos can't refer to the name "RedHat"- they call it "the upstream distribution").
The reason it runs so well on old hardware is that it's an old (aged) distribution.
For Linux, I would choose Ubuntu Server or Centos. Both are going to be rather unfriendly, unless you are experienced with OSX command line tools, in which case you should be pretty comfortable since many of the common common command-line admin tools will work just the same. You can add a GUI (Gnome or KDE, as well as multiple more light-weight choices) but that will soak-up memory and CPU time, so normally you wouldn't install a GUI on a server.
You could also go with a version of BSD, which would be even more OSX-like. Or, why not just go with a Mac Server? (Besides cost.

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(A BRIEF history - Unix originally came in one flavor System 7, from Bell Labs. (I have no idea about System's 1 through 6...) Source code came with, and the University of California at Berkeley forked it to create the Berkeley System Distribution, or BSD, with number of improvements. in the mean time, System 7 somehow got followed up with System III and the System V. (Again, don't ask messy questions bout System's I, II, and IV...) Linux was a complete ground-up rewrite, but is based more in principle on System V than on BSD.
The big difference vs OSX will be file locations and system startup. While OSX uses a BSD kernel, Apple has diverged significantly in terms of where common files are found in the filesystem, and how the system starts-up. Oh, and the GUI, which probably won't be an issue for a server. The OSX GUI is completely different than the X-Windows-based GUI found on these other OSs.
The reason I recommend these particular OSs is that they are widely used and supported. Go with something obscure, and you're going to be awfully lonely trying to figure stuff out on your own.