I'd say Orton.
To me, Orton never quite sold his face-persona to the crowds. I mean, he's only just started getting the pop from the entire crowd again. The boos started to disappear right around the time that people saw him taking on Chris Jericho, and now he's facing another top heel in Edge, who the majority of the crowd can't stand. He's being pushed into a face-spot again, against guys who are natural heels and will probably never be faces again.
Now, before you get me wrong; Cena's work ethic, devotion to the business, and dedication to his craft are top notch. He'll go out and perform no matter what people think of him.
His ringwork isn't that great, but it's not as bad as some of the top guys right now.
On the otherhand, Orton I think, has incredible ringwork. He's slow and calculating, so you can say it does get boring at times; but even Bret Hart was a bit slow in his calculating attacks at times. I think Randy's got the potential if he does go back to being a face, to do even more in the ring. High-flying, suicide dives, etc. is mic-work isn't golden, but he can sell something for a promo like it's nobody's business. And I believe he's ranked just behind Cena in merchandise, for a heel.
I think Orton is more important to the future of the WWE, based solely on his relation to Ric Flair and Hunter. I believe he's guaranteed a spot at the top for years to come.
It's a tough one to decide, but personally, I'd say Orton's a bit more important to the WWE.