When comparing W3Schools vs CheckiO, the Slant community recommends W3Schools for most people. In the question“What are the best resources to learn Python?” W3Schools is ranked 15th while CheckiO is ranked 18th. The most important reason people chose W3Schools is:
All the tutorials are written in a straightforward and easy to understand way.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Easy to learn
All the tutorials are written in a straightforward and easy to understand way.
Pro Built in editor
Almost every example has a "try it yourself" button which opens up an editor in a new tab. It allows you to play with the example code and see how it works.
Pro Well organized tutorials
All of the lessons are separated into their own pages, which makes it easy to learn about specific concepts.
Pro Great source from Google search's perspective

Pro Community backed solution reviews
Users can publish their solutions for different problems and other users can review the code and give their opinions on how they would have solved the problem.
Sometimes, some of the most active users get their code reviewed by Guido Van Rossum (the creator of the Python language) himself.
Pro Available in multiple languages
CheckiO crowdsources translations for problems. Currently problems have been translated in 10 languages. So it's highly possible that eventually more languages will be supported.

Pro Ability to create new challenges
New missions can be created and uploaded using GitHub. After that is done, it's suggested to CheckiO and if it fills all the requirements after being reviewed by a CheckiO team member, it's pulled into the game.

Pro Challenges from GitHub, O'Reilly, Dropbox and Hubspot.
Cons
Con Outdated practices / problem solutions
The practices that are shown to solve the problems at hand are rarely, if at all, updated. Usually, their tutorials and learning material is updated only after they see their profits drop.
Con Doesn't care about teaching right
There are multiple errors in the data they show. Although the solutions they show work, they will lead to unmaintainable code. That happens even when the maintainable code alternatives are as easy or accessible to new programmers as the alternatives.
Con Certifications not recognized
Many professionals in IT agree that w3s certifications are not recognized by them and are deemed useless. Good luck finding any respectable professional that accepts a w3s certification.
Con It is for profit
What defines what goes is and what gets fixed on w3schools is what gives them profit and what doesn't (through their ads system).
Con Written tutorials only
While many learning resources offer a mixture of media in their courses (such as videos, challenges etc.), w3schools offers only written tutorials and code editors. This makes w3schools more beneficial as a quick reference rather than a primary learning resource.
Con Not for absolute beginners
CheckiO does not teach syntax, instead it teaches how to solve problems using a particular programming language therefore some basic programming knowledge is required to start.
