The SNES & N64 cartridges were a standard ROM (with some EEPROM/flash). Because they were a standard they were easy to copy. If they had been propriatory chips the copiers wouldnt have happened. For example the SuperFX and DSP cartridges (StarFox, Mario Kart) were not supported for a long time.
Choose something completely propriatory and you have full control. It will cost more to make though. You'll need your own duplication machines etc.