As just a user of both distributions, I can easily write a comparison of the two systems, while being more objective than the fabled Differences_with_Gentoo.
Both are great distros, don't let anyone make you think otherwise, especially since they are source-based (a key factor imho).
I recently fully switched from Gentoo to SMGL and my rationale is very simple. Sorcery (the Portage counterpart) is simply faster and more featureful. And it's written in bash! Do you feel the tingling sensation? 
At first I thought the bash part was only going to make it slower, but the results show a different image.
Speed example: searching descriptions. If you ever used emerge -S you know that not only you can get a cup of tea while it scratches, you can even plant some in the time it takes to finish it's job. gaze search on the other hand is done in ~15s (7200rpm, 8mb cache).
Featurefulness example: triggers, ressurection, healing, parallel downloading and default ccache are the ones I can think of at this moment. Just think how broken or semiusable are apps like equery, etcat and qpkg - gaze beats them by a mile. A nautical one.
Gentoo does have some benefits over SMGL:
more users -> documentation
- config update merging tool
- kde split ebuilds could be cool too, but it doesn't seem like the idea was carried out as planned (yet) though.
- approximately twice the package count
- more eyecandy (think prompt coloring and gensplash)
And other stuff I don' care about:
- supports more platforms
more packages -> more choice (think init)
The key difference is that the gentoo bs (build system ;P) is noninteractive, while sorcery isn't. It takes a while to get used to it, with unknown results. Sorcery will also not completely break the cast if one package (out of many) doesn't build.
None of them provide any default means to view the messages at the end of builds. You can use enotice on gentoo and gaze compile on smgl (you don't know when to do that though).
...
So for me, it was a simple decision. It depends on you and your needs what the better choice is. Choice is good, so centralism isn't. There will never be a globally perfect distro, so there is no point in forcing your views on others (plain fanaticism)... /bla
I'm sure that as time progresses and things evolve, I will find more (dis)advantages regarding both distributions.