Mugen actually is a more developed fighter engine, due to the KOF91/95 is the past version, Mugen evolved out of KOF, so KOF is good, it runs ok and such *havent played much due to last time i had it sucked XD*
Basically we do not need mugen due to we have the predesecor of it
Yeah, MUGEN gives you about 100 times the freedom and customizability of KOF91 when it comes to programming character-specific states, special effects, helpers, velocities, etc. Which, of course, makes half the available characters RAM hogs (some are >10MB!)