I have tried both of these guys, and I know them pretty well so here are some of my opinions:
Admin interface: both are simple and kinda similar. CMSmadesimple is a little more complicated and the way it handles pages is uglier. In order to create new page you have quite some forms to field. This is a breeze with WB.
I would say nearly a tie, WB just a little better.
Template making: both are straight forward. CMSmadesimple allows edit everything in the backend in an easier way than WB but they use Smarty engine. Might be pros and cons for some, for me it's cons. I prefer pure php syntax the way WB does.
WB wins.
Modules: both have basic modules but due to huge fan base, CMSmadesimple has more modules. However, both lack a good blog module (is it that hard genius developers?

).
CMSmadesimple wins.
API and how to develop new things: I have no ideas since I'm mainly a designer. Some said CMSmadesimple has very good codebase and API - I will leave this blank.
Speed: Back-end WB obviously performs faster. Front-end is kinda the same. I don't know much about WB caching but CMSmadesimple devs are very proud of their caching technology. (I posted a question in their forum about this to ask already).
Future: It depends on you guys - WB lovers. CMSmadesimple is now very popular and won quite some awards from Packtpub in 2007 and 2008.
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Final words: I see no reasons to move to CMSmadesimple for small-mid sized sites. If I want something huge, I might try typolight, joomla... If I want something sexy and slick, I will try Modx, Silverstripe, Concrete5.
Most of my websites now and will be published with WB - I definitely love it - and the community.
Hope my 2 cents help!!!