Answer 1

Do what intrest you.
Challenges: Test your knowledge
Coding: Practice and solve problem

Answer 2

https://www.sololearn.com/Discuss/704438/?ref=app
https://www.sololearn.com/Discuss/683550/?ref=app

Answer 3

If your goal is learn then go with writing codes. If you point to xp (and is NOT same like learning) go with challenges... You can do either also