well good choice on considering minidisc as an alternative to an mp3 player!!
First of all, a minidisc stores music according to minutes, not megabytes. A minidisc can hold 80 minutes of SP music, 160 minutes of LP2 music, or 320 minutes of LP4 music. SP has the best sound quality (near CD quality), followed by LP2 and then by LP4. The sSound degration between SP and LP2 is not noticeable on a portable such as the N707, but a HiFi unit, it's noticeable.
Inless you're an audiophile with Ety's or something, an N707 with LP2 discs seems like the best thing for you .