As for configurability KDE has quite a few more years under it's belt, so it's not a surprise, but GNOME 3 is all JS and CSS so I would say it's pretty configurable and the number of extensions is only going to increase. I think extensions will become much more stable around GNOME 4.0, when they come out with a full OS [1]
[1]http://www.phoronix.com/scan.php?page=news_item&px=MTE0ODg
KDE has to many buttons for me :)
I think it's more of a desktop for people who are use to using a lot of advanced features on Windows, because they're used to and enjoy the idea of having a button for this and a window and sub-menu for that. GNOME is more for people who either don't use advanced features, or use the terminal for advanced features.
Personally I'm over the traditional desktop.