As Herbert has commented, this is a question of "which car is better", i.e., the answers are bound to be subjective to some degree. Also, Stefan has already aptly summarized the topics "File structure" and "Documentation". However, I'll add my two cents with regard to "Integration in the LaTeX world".
Imagine a scale from the LaTeX standard classes to a hypothetical class that includes every feature described in the LaTeX companion (plus a collection of "Best LaTeX packages developed after 2004"). Note that the latter is the model envisioned by the LaTeX3 team. On this scale,
Yes and no.
To delve into the realms of subjectivity, here are some features where I feel that
These points and others led me to believe that
Both bundles are allround packages which work together with many external packages. Both are very good for producing well designed books, much better than base classes. Here are some differences.
So I prefer the finer KOMA-Script bundle structure. The memoir approach is ok if you just use it as it is.
So memoir wins regarding English. KOMA-Script for German. It's a hard choice if you understand both languages. Then I recommend looking at the published KOMA-Script book.
Integration in the LaTeX world
So KOMA-Script offers a wider contribution for TeX users, while the memoir approach suffers if the original external packages would be further developed.
Both KOMA-Script and memoir are great packages. It's still a hard choice if you focus only on book writing. However, if you plan to write other documents than books, such as reports, articles, letters, than I think learning KOMA-Script is more beneficial than learning memoir.
Note by lockstep: KOMA-Script v3.11, released on May 15th, 2012, features a new English manual (though it is "still a work in progress", Preface).
"Your mileage may vary"
These aspects of memoir make it, for me, a better choice than komascript:
Now, I should qualify these comments. Both are very large packages, and require a significant investment of time to learn completely. I came across komascript second; by then, I had already invested a lot of time in, and was very happy with, memoir. So in some sense, that disqualifies me as an unbiased critic.
Beside writing articles, reports, or books, KOMA-Script also provides writing letters. The KOMA-Script letter class is the most flexible (the authors say: "versatile") letter class I know. There are even pre settings for letters using Japanese envelopes. And at his homepage the author offers an extension for sections at letters and shows how to define your own notepaper. At the appendix of the German KOMA-Script book you may also find a very large notepaper example.
Nevertheless, some of the default settings of the letter class are unusual, e.g., you should change the alignment of the signature:
But you have to do this only once: Save this changes together with your sender information and notepaper settings in a file called lco-file and load this file via class option.
I've made notepaper files for some german companies. Most of them basing in asymTypB.lco from the KOMA-Script author. I needed about one day for the first one. But after learning the general concepts of variables, pseudo-lengths, and moving elements around I became much faster.
In case you really wish to design a graphic layout for a book, ask here in this forum or anywhere else, how to realise certain of your ideas in LaTeX. KOMA-Script (and probably Memoir) are not about fancy layouts.
KOMA-Script provides many shortcuts to deal with headings, captions, paragraphs and anything you need to write a book. It is very stable and reliable, bugs are corrected in short time; the maintainer holds interaction with other packages in high regard.
That said: you will have to write some lines of code with any documentclass to have e.g. pagenumbering in a rounded blue box or thumbs with chapternumbering on the outer margin.
Years ago I chose KOMA-Script. It simply worked when writing under pressure (other than word, but that's a different story). Never had regrets. Great software.