Coinciding with the Disk System's release, Nintendo installed several "Disk Writer" kiosks in various toy and electronic stores across the country. These kiosks allowed customers to bring in their disk games and have a new game rewritten onto them for a ¥500 fee; blank disks could also be purchased for ¥2000. Nintendo then decided to make an early form of online gaming; In 1987, they introduced special high-score tournaments for specific Disk System games, where players could submit their scores directly to Nintendo via "Disk Fax" machines found in retail stores. Winners would receive exclusive prizes, including Famicom-branded stationery sets and a gold-colored Punch-Out!! cartridge. Nintendo of America announced plans to release the Disk System for the Famicom's international counterpart, the Nintendo Entertainment System, and began filing patents simultaneously. However, by the time these were approved in November 1988, Nintendo cancelled their plans to release the system stateside.
Castlevania 3 sounds quite better on FDS indeed, but cartridge Metroid is by far the worst case of this in my opinion.
Part of the soundtrack is missing channels that contribute to the main melody. Even the now iconic item fanfare was barely recognizable in that version.
Also of course there's the fact the FDS Metroid had actual saves on the disk, and they just didn't implement them in the cartridge version, unlike Legend of Zelda. Instead we got that painfully long password system.