The Maximus VI is asus's ROG (Republic of Gamers) board, it's reserved for their flagship hardware. this means it comes with extra bells and whistles
From what i can see is the following
intel gigabit server grade LAN adapter
An additional SATA controller supporting 2 additional 6gb/s drives
Higher end AUDIO card (Supreme-FX)
Power, Reset, Bios Reset buttons on the board
It will also have points for voltmeter to read voltages for CPU & RAM
Realtek Gigabig LAN
None of these features increase performance.
little bit better sound quallity (unless you have awesome speakers you wont notice it)
these ROG boards are designed for ULTRA high end systems and designed to make Liquid Nitrogen overclocking easier with the voltmeter points & motherboard buttons.
Simply because your asking your self "whats the diff" means you will not require the features of the ROG board.