Both are good and there are many people n sololearn who had taken benefits of w3 schools.
The main difference is that sololearn is an online community driven platform with some awesome courses designed especially for beginners.
w3schools has in-depth tutorials with exercises for each lesson.
My suggestion is to try both of them
Dark Code Cat sololearn is best beacause sololearn organize many new things like contest challenges to improve your coding skills also you can ask your coding related questions
You can also share your coding related tips and there are also much more things on sololearn to do.
I think you can't find the app or website which is better then sololearn
I think that we haven't to compare them. 'cause they haven't the same learning methods.
-W3S is based on learning sequentially like a full and clear documentation.
-SoloLearn is based on interactive and community learning.
For that we have to use both to improve yourself. Since it's two differents methods.