Again, I'm not an expert, but I think it's better to learn on something that you can hear sounds well with (more loudly, clearly, less muffled, less background noise) because pathological heart, lung, bowel, & vascular sounds are often pretty subtle. Once you know what crackles, bruits, ejection murmurs, etc. sound like, then you can probably hear them better with any cheap-o one you happen to steal off a med student as a resident. I find it hard to believe that you'd spend years with the benefit of clear auscultation and then pick up a crappier steth and are suddenly crippled in physical diagnosis. That said, I guess I wouldn't go for a stethoscope that has more vast technological/structural deviations from everyone else's stethoscope (i.e. the electronic ones, though some may disagree).
We can get into an amateur discussion about whether a medical student really needs to be able to appreciate those subtleties & the diagnostic value, but I'll just say I can hear much more clearly with my nicer scope (Cardio III) than with a few of the cheap-o ones I've tried, and I'm not alone.
Does it matter that much? No, stethoscope quality & function is a spectrum like most things, so if you get a "decent" one I'm sure that's good enough. But I'm glad I have a "good" one without a doubt.
Also - if anyone says the "the most important part of a stethoscope is between your ears" line I'm going to throw myself off a building in exasperation.
