I very much agree with 12345j, creating templates is really clean and easy with concrete5. A basic concrete5 theme has 4 lines of php: one to include header scripts, one to include footer scripts, one to create an editable area (like a widget area) and one to display it. To add further editable areas, you add the same two lines.
With Wordpress, theming is very well documented, but there are a bunch of different files for the posts/categories/comments/etc. I know wordpress themes can be very simple too, but I'm convinced that to a newcomer it's more complicated than concrete5.
Areas in concrete are way more flexible (and easier to understand) than Widget areas. When I first started using Wordpress I couldn't believe I couldn't easily set up different sidebar areas for different pages - it took extra plugins and config.
There are so many things in concrete5 that I've found that solve typical development 'problems' that other systems don't handle well. In particular, the Composer, custom block types, page attributes and block template overrides. All these things come into play when you have a client that wants to adjust something or have an easy way to add content. They're things you might not see straight away with the system, but when you find them, you're really glad they are there.
If you are a programmer, it's an incredibly powerful and well designed system. You don't have to learn at all at once too, you can learn how to create 'block' templates to tweak up some output, move on to overriding block functionality and then on to creating your own blocks.
So I'm very happy using concrete5 - it's easy for clients, easy to theme and highly customisable.