I’ve been using Avast free till now. Are there any reasons to switch to Windows Security (formerly Defender)? Which is better as of recently?
The answer depends on where you ask your question. 
There are people here who will give their honest and unbiased assessment notwithstanding this being the Avast forum.
I have 4 computers all running the latest version of Windows 10 (20H2)
All systems are protected by Avast.
There really are enough sources for you to make your own decision, AV-Comparatives for example.
Whilst MS defender has improved its detection rate it doesn’t cover as many areas as Avast does, even the free version.
Plus (AV-Comparatives) don’t test all areas of Avasts protection.
MS defender also has a high number of False Positives in the AV-Comparatives tests, which can happen when trying to improve detections.
That said there is only one person who can make that decision for ‘you’ and that has to be based on your investigations and your requirements.
Avast Free suits my needs and I have been using it without infection for over 16 years, so I guess you know where my answer lies.
Have you tried Wilders security forum https://www.wilderssecurity.com/
What’s special about it?
Wilders Security Forums is a Community Based Discussions board about Online Privacy, Security Programs and Data Protection
This is not a new question. Previous chat about can be found here if you are interested:
https://forum.avast.com/index.php?topic=222229.0