I almost exclusively use the ball of the foot when sparring. Occasionally I use the shin for light leg kicks or the instep for kicks to the head but I personally prefer the ball of the foot when barefoot. It definitely has power for me, but you won't know what works for you until you try it 1000+ times against various opponents
As for barefoot. Can I throw a ball of the foot without pulling my toes back and still be OK when it makes contact in sparring?..isn't it all down to angle of the foot, and less so about toes pulled back?