Actually, I have no idea as my experience with Wordpress is new.
One thing ( and I know nothing about CMS's ), how do newcomers like Concrete 5 etc look in terms of competition. It's easy for developers to migrate from one to the other I guess, and there are likely others.
An agency I spoke to today told me they and a few others in the area are happy with Concrete 5 , when I asked why they preferred it to Wordpress. The answer was not specifically one versus the other, just that they used another.