If I had to be honest, it won't be just one of them that's the face of the company, it will be BOTH!
I think they have enough of a fanbase to keep level-pegging as the face of the company. Remember back in the Attitude Era, they didn't have just Austin as the face of the company, there was Rock, Taker, Triple H, Mankind. They have multiple stars that provide the face of the company and some having more higher profile than others (Rock and Austin for one). That's why I feel it won't be just one of them, it will be both.
I do expect a one-on-one Wrestlemania match at some point, just feel they again rushed the first time encounter at Summerslam 2007. But then Rock and Austin had 3 WM bouts, plus other matches outside of WM. I expect one for Cena & Orton, possibly around WM 27/28 if I have to be honest.