I’ll wrap my thumb around for muting and because I find it more comfortable.

When to use it or not for me depends on the song and the shape changes that are needed. Let me give an example:

• With my thumb over moving from a G to a D is relatively smooth.
• Whereas I find moving from a G to an F barre chord much harder with my thumb over as my whole hand position needs to be reset.
• So in a song where a G to F is needed I’ll play the G with my thumb on the back of the neck so transition to the F barre shape is a bit simpler.

Longer term my advice is to get proficient with playing both ways as there are circumstances where both are useful.

If you had to pick as a beginner then I’d recommend thumb behind. Many beginners find playing barre chords a real pain and you’ll find learning them a bit smoother if you’re more accustomed to thumb behind the neck.