basically the only difference is clock speed. the qx6800 runs at 2.93 ghz and the q6700 runs at 2.66 ghz.
the qx6800 uses 130w vs 105w for the q6700. basically to push the higher speed. instruction set, architecture and so on are identical.
so if you're not gonna oc the chip, the qx6800 gives you a bit more speed.