Crossfire is ATI
SLI is Nvidia
Crossfire lets you use intel chipsets
SLI -forces- you to use nvidia chipsets (which suck)
Crossfire scales a little better than SLI when it works, but SLI seems to have more support than crossfire.
You can however there is no support for SLI on that board, only crossfire. If you wanted to go nvidia you could use two cars but in stand alone mode only.
I dont know bout you guys but i will stick with nVidia and only nVidia i never did like ATI cards so if i cant have sli on the maximus i guess in the future i will just get the 780i board.