Don't want to get into the percentages since none of us believe Cena has any shot of losing. That's the problem.
The thing is with the current Cena dynamic, he's been to the mountaintop. He's been an 11/13 time WWE Champion, he's always played up as the underdog even though he's actually the odds to overcome, the top guy. But you already know this.
Outside of the little kids who love Cena with every bone in their body, the story here is the guy who's been around for so many years who's never held the WWE Title vs the guy who's been on top for so many years. Mark Henry should be the sentimental favorite despite being the heel. If Mark Henry wins semi-clean you actually have a really good story with that the adults (who don't like Cena) will enjoy. Being a heel you can argue that shouldn't happen, but this is Mark Henry we're talking about. He'll make it work.
There's a lot of history here too. Ratings haven't been good this year, and the last time WWE had a big ratings increase was when? Smackdown in 2011. What turned around the ratings? Ratings Henry. Similar buildup, too. You have a top face (Orton/Cena) who has a one-note character, getting heat from much of the audience after his win over a crowd favorite (Christian/Rock) for backstage reasons that irritate the IWC, then goes on this reign of terror and nobody feels like they could ever lose the belt. Then Randy Orton jobbed CLEAN to Mark Henry, and not only did ratings (and show quality) improve, but Orton became the most over he's ever been. Why? Because Mark Henry's the best dominant heel we've seen in years so Orton finally had a legitimate obstacle to overcome, although it's up for debate with how things transpired after that.
Henry doesn't have to win 100% clean, but it should be decisive. You can have Cena make one little mistake and give him a "I was careless" excuse, or have him bounce off the exposed turnbuckle and into the WSS, or something like that. Giving Henry a thank you reign is good for the fans, good for the talents, and good for business.
But with this, Bryan would be stuck in no-man's land.