Hello mitchm and welcome to the SmaugMuds forum.
I'm going to start by saying that comparing Smaug to CircleMUD is like comparing apples and oranges. While both are Diku and Merc derivatives the similarities end there (This is just my opinion, someone else may think differently). I played on a CircleMud for years and one day tried a Smaug mud and I was hooked. Maybe it was the fact the Circle I played was done in almost a completely boring grey colour and the Smaug introduced me to a world full of colours but it was also the fact the Smaug mud appeared to be easier to play. Not easier in the sense that I could kill a hundred mobs without taking a single hit because it wasn't, but easier in the sense that it somehow felt more interactive, lighter, less 'clunky'. I'm not sure how to explain it but again, this is just my opinion.
On to the basics and differences.
Smaug - the original code base
-> SWR (Star Wars Reality) - a pre-themed version (yes, it has spaceships!)
-> SWFotE (Star Wars Fall of the Empire) - a pre-themed version
(FUSS = Fixed up Smaug Source) - to quote this site "The ongoing goal of The SmaugFUSS Project is to find and eliminate as many of the bugs in the stock Smaug source code as possible so as to provide for a clean, stable base for new admins to run a Smaug mud from."
is an advanced C++ derivative of the Smaug MUD codebase (C).
(Lands of Pabulum) is a feature rich version of SmaugFUSS 1.7 - newest and latest features.
All are enriched versions of Smaug and all are actively supported in these forums.
CircleMud - the original code base
-> probably many pre-themed versions
is Circle with Extras. To quote their site, "...that has many features which are not typically found in a publicly released MUD."
is Circle with Goodies. To quote their site, "CWG is a project which is based on CircleMUD 3.1. Our goal is to create a stable, newbie friendly distro which already has many common features patched in."
Basically it's to Circle what SmaugFUSS is to Smaug.
-> Buddha, SunTzu, Rasputin, from what I found in google are pre-themed CWG codes.
There's also tbaMUD
- "the codebase formerly known as CircleMUD."
All are enriched versions of CircleMUD and appear to be supported at http://cwg.lazuras.org/
Choosing a code base between Circle and Smaug will be a matter of preference. While working with the code is similar there will be differences in features. My suggestion would be to try all of them, see what you like, what you don't like, does it have what you need, is it easy enough to upgrade and add new features, etc. The newest ones can be determined by the date it was put in the code repository (although newer may not always mean better) and of course the latest versions can be found by version number.