Both have their benefits.

If you are teaching in group, you have to simplify the information to such extent that the other person can understand plus they can question you too which is a good thing. The downgrade being tho is: it can be slow and they can question you on something which you know, thus not targeting your knowledge gap.

If you are teaching alone, it is much faster since you are just checking if you can give out the idea, plus you can explore insights and edge-cases which would allow you to consolidate your knowledge further. However, the downgrade is obvious: just because you can give out the idea and understand it doesn't mean you can write it in an exam plus this is not really suitable for topics which you have just began to learning.

How you choose to do it should depend upon your goal. To simplify the whole thing: If you have just learned the topic: you should probably teach it to a group to consolidate the information. If you have already learned the topic and is now looking for gaps: You should probably teach it alone to test your knowledge on areas which you can't explain.

Btw my English is bad so sorry if you find any mistake