One of them will win the Rumble, bank on it.
I'd bank on Punk. He's rumored to win it next year and between being in a hot feud with Paul Heyman and no doubt moving on to feuding with the HHH regime next (which means a WWE Title match against Orton at some point, probably), that guarantees that he'll be involved in hot programs from the time he returned in June to the Rumble next year, and a Rumble victory could be the ultimate thing to top it all off.
I doubt Cena will win it again because they already did this "make an unannounced surprise return at the Rumble and then win it" with Cena in 2008 and I don't see them repeating that exact same scenario again with the same guy. I can't see Sheamus winning it again either. I think the winner of the Rumble that's part of the road to the big 30th anniversary WM should be a fresh winner, not one of the previous two winners.
I personally think Cena should return before the Rumble and announce his entry into the match to build up anticipation that Cena could win it to start his comeback off strong. Unlike in '08 when people were surprised to see him come back so early, we all know how many months it'll be before he's cleared to return so the surprise return wouldn't be as effective as it was six years before.
I think it'd be awesome if the match came down to Punk and Cena. Imagine the crowd reaction for that kind of stand off. The commentators could play up their history and go on about how only one of them can win and go on to Wrestlemania, yadda yadda... And it ends with Punk tossing out Cena. Or it could also be interesting to replay the Cena/Batista '05 scenario where both guys hit the floor at the same time but then the match restarts and Punk gets the definite win then.