Well most mechanical keyboards are ODM'd by Costar. Including the Mionix as far as I know. It boils down to materials used to make the major components and keycaps.
In regards to what makes it better...
I personally dislike backlighting as it's pretty pointless. Especially if you're a touch typist. Then there's the technologies used to do the backlighting. There's a dissipating sheet with just a few LEDs or each key has an LED soldered into it. Either way, it's just one more thing that'll be a pain in the ass, or expensive to fix, should it go wrong.
I prefer the simplicity of the Filco and similarly designed keyboards.