I hadn't seen simpla.io before - the demo seems very nice though and the concept pretty cool, their mechanism for managing the logic behind editable regions seems really clean.

To clarify - ContentTools is not a CMS - it's a front-end tool that provides a more natural editing environment for HTML pages than traditional rich text boxes. You still need a CMS behind it. What I've presented in the Saving Strategies tutorial is really intended to help those looking to integrate ContentTools into their CMS understand how the output of the editor can be stored. The example code in the tutorial could I suppose serve as a basis for someone looking to build there own CMS but I suspect in most cases it's far too simplistic for that purpose.

So coming back to your original question I guess the main difference is that simpla.io provides both the editing environment as well as an API for storage to different platforms (such as dropbox which they mention on the demo page). ContentTools only provides the editing environment - the storage of content and other CMS responsibilities are out of it's scope.

For storage simpla.io provides a remote API which you either need to sign up to use, or which you need to host yourself (you can host there API locally). The simpla.io front-end library sends updates to this API which is then responsible for storing the content somewhere.

Reading the questions at the end of the simpla.io KickStarter page it appears you still potentially need a CMS, e.g to manage the creation of new pages, access control, etc.

So really only the two editing environments are comparable and the best way to establish the difference between them is to try the demo pages on both sites (which I'm sure you have).

Thanks for your question, I hope my answer helps better define ContentTools' role.