On the WindowMaker site, there's a page that has a good comparison of a number of different window managers. It looks at memory use and load time as well as feature sets. The data is a little old (2003), but it's still a useful table. The big desktop environments like KDE and Gnome aren't included.
The interpretation on the page leans toward WindowMaker, but the data in the table is objective enough that you can derive your own conclusions, I believe.
Here's the link:
http://www.windowmaker.org/features-performance.html
