I have the Littman Master Classic II and the Littman Master Cardiology. These are some excellent stethoscopes. Whenever we land on the scene, our patient is usually waiting in an ambulance where we take a chance to listen to their breath sounds, etc. Given that noisy environment, it is quite important to have a good quality stethoscope, since we cannot hear anything with our stethoscopes in the chopper.
I would recommend the Master Classic II, it is the cheaper of the two. You can get one for around $100. The Master Cardiology runs about $200.