The one you can wear pretty much anywhere. The Midi gives you that lovely feeling of having more hair, without it becoming too much to manage day to day.
Two inches makes a real difference here, so use the Size Guide and a soft tape measure from your crown rather than choosing on the number alone. All three caps are available at both lengths. Choose Blunt & Straight when you want it simple, or Layered & Wavy when you want more softness and movement.