It's Horwill's spot to lose. Even in average form, he showed enough on the EOYT to prove that he can be a handy member of the squad, albeit only a bench / injury back up. He has a tonne of experience and his set piece work is strong. If simo goes down injured, I would think Horwill would be the most qualified to run the line-out.
I would be shocked in Rory was in the RWC squad. Just too inexperienced. There's only 4 games before the world cup to blood new players and I would think Cheika will want to be settling on his top side for at least 2, maybe 3 of those games. Barring injuries, or some shocking form from horwill, I cant see it.