"The main difference, from what I can tell, is that the diaphragm picks up sounds (that is, the diaphragm is excited by vibrating air) while the bell is directly excited by (acoustic) movement of the skin."
This may be helpful also: http://science.blurtit.com/15743/how-does-stethoscope-work
Commonly available Acoustic stethoscopes operate on the transmission of sound from the chest-piece, via air-filled hollow tubes, to the listener's ears. The chest-piece usually consists of two sides that can be placed against the patient for sensing sound — a diaphragm (plastic disc) or bell (hollow cup). If the diaphragm is placed on the patient, body sounds vibrate the diaphragm, creating acoustic pressure waves which travel up the tubing to the listener's ears. If the bell is placed on the patient, the vibrations of the skin directly produce acoustic pressure waves traveling up to the listener's ears. The bell transmits low frequency sounds, while the diaphragm transmits higher frequency sounds. The problem with acoustic stethoscope is that the sound level is extremely low, making diagnosis difficult.