A third: they are permenantly protected in a sealed plastic case with a spring loaded shutter. The whole magilla gets loaded into the player and you never come into contact with the disc. So compared to CDs, MDs are virtually impervious to damage when you’re on the go.
Ah but remember, MDs weren’t meant to be competing against CDs. Make the appropriate comparrison (MDs versus cassettes) and then tell me which has better sound quality.
But even if we do compare MDs to CDs, the difference is not audible to 99.9% of human ears. I was an audio tech working for a subsidiary of Sony at the time they were released, and had the opportunity to listen to both side by side. Only in one case out of dozens of samples could I detect something different (not necessarily wrong or missing, just different) between the two, and only while examining the output with superior quality mega-bucks headphones. And in at least one case, when they played a CD, then switched to MD I imagined a difference when there was none (they actually switched back to the CD, causing me to think that I was now listening to the MD- there should have been no difference. Kind of like asking you to choose between two unmarked sodas, and when you make a definite choice and say that one is clearly better than the other, only then do they reveal that they are both Pepsi). In an ordinary room with your average speakers (which, all by themselves introduce 5% or more THD), there was no audible difference.
I still have the VHS tape of Sony’s first commercial attempt at marketing the MD. It is full of people swearing on a stack of bibles that they couldn’t tell the difference (any that could tell the difference were edited out I’m sure).
On the oscilloscope it was another matter. With special test discs I could see the difference in the measurements every time. I suspect that comsumers, being naturally cautious about spending big money on a new format, looked at Stereo Review, Audio and other popular electronics review magazines and saw that the MD output showed a measureable difference compared to CDs. Consumers then said “ah HA! MDs are inferior to CDs!” Never mind that you couldn’t tell by listening, that was the beginning of the end for MDs.