Crossfire. While SLI does scale better, crossfire is more forgiving, AMD has eyeinfinity, and AMD has DX11
. For example, you can have 3 or 4 of just about any Radeon card that has two crossfire connectors, even a 4670, but on the nVidia side only the GTX 260 and above will allow for more than two GPUs. On the AMD side, motherboards supporting SLI are a little more problematic than boards supporting crossfire. On the Intel side, boards supporting SLI cost extra for the privilege.