Which head of the bicep is predominantly activated is more determined by elbow position/angle in relation to the shoulder. I will say that both heads are always activated during the flexion motion - its just that one position may activate one head just a little more or less than the other...
Elbows in front (like preacher curls) will activate the lateral (outer) head slightly more. Elbows at the side (barbell curl - if you keep your elbows still) activates both heads. Elbows back (incline curl - again if you keep your elbows still) activates the medial (inner) head slightly more.