I have the Cardiology III and LOVE IT! IMO, the Master is very heavy and the diaphragm is large...harder to listen to my small little elderly/pediatric folks. With my Cardio III, I can use the pediatric diaphragm and it works great! (Plus, it is lighter than the Master, which is an added bonus.)
I LOVE my cardio III! Can hear very well with it and I like how it looks I also love that it has a pediatric diaphragm. I occasionally get floated down to the ER so at times I will have to admit/assess pediatric patients, and that diaphragm really comes in handy!
I've had both. Actually, I've got a Littman Light and a C. III and I can definitely say that I really like my C. III. I find that having a smaller diaphragm sometimes helps me hear because that large diaphragm is sometimes just a little too big to use on smaller patients.