Heel Cena, because people are dying for a character evolution, and I seriously don't get why people are saying Face Cena is a bigger threat to the Streak. He's ONLY a realistic threat to the Streak if he's willing to do whatever it takes, break rules, pull tights, low blows, etc. When Triple H can't beat him in a HIAC after the amount of shit he did that should've ended the match, Face Cena is helpless. All he'll do is put up a great fight and lose. If he's not willing to do ANYTHING, you can write the match off right there.
Yeah, he's gonna lose anyway, but considering the fact that Heel Cena would be more than happy to hit Taker in the balls with a steel chain, don't kid yourself into thinking that he's not going to be seen as a bigger threat.
And besides, if they were going to end it, he would HAVE to be a heel because the fans would try to burn down the building when he won, so him being face would be stupid for that match.