Seems like Cena/Rock is a bigger deal to me, even a third match. (Hollywood>UFC, not to mention Rock was a bigger star previously in WWE than Brock was by far.)
Rock retaining but then losing it the next month would be a swerve and would be quite welcome, but from a story telling POV, I think it's best from them both to walk into their third (and final) match tied at 1-1. I remember Kevin Nash saying last year that Triple H beating the Undertaker at WM28 wouldn't mean anything because it would have still taken Triple H three times to finally beat Taker. I think the same logic applies here with Cena finally getting the victory only on his third try. Plus, Cena would have to qualify as the #1 contender some other way, because if Rock lost, he could easily just use his rematch clause, but Cena would have no such thing.