One man's opinion, the ATRAC compression scheme is far superior to even "great" MP3 compression. Sony got it right. But compression is compression, so if there is a DAT or JB3 copy available, that would be the more "pure" source.
But at the heart of this discussion is "what makes a good tape". Some people would say that no matter how good an MD tape can be (think FOB), it'll never be "pure" enough to trade because it is MD sourced.
I think the MD was a good tool for newbies to learn about taping with. It will teach you to watch your gear carefully, with the fear of dropouts and the need to flip tapes. It's cheaper to get an MD deck than jumping right into a DAT, and if you don't like it in the end, you still have a nice compact portable music player. However, even with the new Hi-MD format coming out, I wouldn't reccomend newbies go with MD. The JB3 seems to be the better choice for todays novice.