Rory's standup game is subpar. Even if he takes Nick down, he can't do much because Nick's ground game is top 3 in the entire sport and Rory doesn't have the massive size and strength advantage that GSP would have over Nick.
Take it as you will, I think Nick would beat him. Either by pitter shatter or some type of submission. Unless Rory runs all fight and makes Nick disinterested in fighting anymore.