The biggest difference between the WWE Champion The Rock and John Cena, besides the victory at WrestleMania 28, is simple: The Rock knows he can beat the greatest marketing tool in WWE history. Cena, like the Little Engine That Could, thinks he can beat one of the most popular wrestlers in the history of the business.
No matter how easy that may be to comprehend, it is more significant than just a "People's Elbow" or an "AA" as a finishing move. The entire dichotomy of the match and the outcome is predicated on the fact the WWE needs Cena to win this match, not whether he should pin "The Brahma Bull" for the victory.
This match is a matter of righting a problem the WWE is now facing with a part-time champion who may be scheduled to work through Extreme Rules next month.  The company is getting nowhere with a belt that is not featured every week on Raw. Booking The Rock to win the WWE Title may have been a stroke of brilliance with the Royal Rumble and the release of Dwayne Johnson's movie, GI Joe: Retalliation, but it is awful for the wrestling scene.
The WWE title has never meant so little in the eyes of the company, where it seems Vince McMahon is more concerned with the "Entertainment" part of World Wrestling and forgetting what got him on the map. That is where the differences between the two opponents for the title match began and continue.
The Rock is a universal character, traced in movies, commercials, television, etc... Cena has had his day in those mediums as well, but his roots remain firm to the company that made him a star (and currently one of the more hated people in the business if the crowds in Washington Monday night are any indication.) He spoke of it last year when the two met at WM 28, and while he has not been as aggressive in his attack in that realm, the same still holds true.
Yes, The Rock won the title and has been on television more this time around, but it is still not sufficient enough to warrant a long title run. For whatever we hate about Cena, his body of work this year warrants the title, plain and simple. The Rock is more there as a publicity stunt, a way to create drama for himself and hopefully a quick fix to the problems that exist within the WWE.
The Rock gets people in the seats. He is an exciting train wreck waiting to happen with his opponents, and he gets his message to the millions (and I mean millions) of WWE fans worldwide.
But he cannot fix writing, planning, booking, angles, injuries and continued appearances on Raw every week.
Cena cannot do any of that either, except show up every Monday night and hear the chants we all love to hear. "Let's go Cena. Cena Sucks!"
If you want to take this a step further, it is the slippery slope Ric Flair used to run from time to time in the NWA and WCW.
Flair was the company's greatest villain, but even with his "mean side," he was still loved and hated at the same time. It's difficult to hate someone you want to drink a scotch with and ride Space Mountain with (ladies) all the time. Fans still hated what he said and could willfully root for and against him.
Cena is a hero to the kids of the WWE Universe, wins, says the right things and is the Hulk Hogan of the WWE right now. But on the flip side, he is disliked for his "vanilla" exterior by adults and his often "Wally Cleaver-like" comments. Members of the military love him and the fans of an "edgier" generation want someone like CM Punk to guide them.
It's a no-win situation. And Cena and The Rock are caught in the middle of it. But for now, the WWE needs the "softer" of two sides to prevail. It's the only way to make the title that made both of these men superstars important again.