I'd go with Cena. Everything he does is interesting, and can be made into a big deal. Just look at the feuds with Umaga, Khali and K-Fed. He has more versatility as a face, and the fans are so sold on him, they would react to anything he does.
Orton is pretty good too, but I don't think he'll be as great as Cena. Just look at both of their recent injuries. IMO, Cena's return was more important and meant more to the crowd than Orton's return. Orton has been given lots of opportunities to succeed. Some of them have been amazing, like what he's doing right now. Others have been ruined, either by him messing something up, or not enough interest.
Orton is pretty good too, but I don't think he'll be as great as Cena. Just look at both of their recent injuries. IMO, Cena's return was more important and meant more to the crowd than Orton's return. Orton has been given lots of opportunities to succeed. Some of them have been amazing, like what he's doing right now. Others have been ruined, either by him messing something up, or not enough interest.