I would guess nvidia. They are easy to use in 4 way and 8 way configs , some renderfarms are using them in 4 and 8 way configs. So if you divide a screen into a matrix of squares it becomes really easy to have all the cards working together to make one picture.
This is not sli like people are used to on the pc, but instead like putting lots of monitors on a wall with each set of cards running each monitor.