I like to incorporate both in my routine.
Like someone said above I believe that preachers work on the peak and bb curls work mainly on thicknesss and width.
No.... they both do the same thing. As the bicep shortens (contracts) it raises the lower arm up to the upper arm in a semi arc. Preachers will force one, to a degree, to do the movement strict by not allowing body swing or poor form thus putting increased emphasis on the arms (biceps).