Sado or Yachiru

Animatri

I've been wondering, but which one is actually better?

I have both, but i am currently useing Yachiru since i heard she was better, but might Sado actually better? Is it better to have a tank then a dodger?
Since i see that most people who are ranked above me has Sado.

I'm new to this game so i havn't fully understood the system yet.

Also, why is SWP partners bad compared to Tavern partners?

Thank you for your replies