In 2013, Cena signing with TNA would definitely be bigger since Cena has much more to offer any wrestling product nowadays than Hogan does. The real question is, would it be bigger or as big as when Hogan signed with WCW in 1994? To be honest, I think it could be. There's no dream match similar to Hogan/Flair waiting to happen for Cena if he does make the jump, but he's currently the face of a global brand like WWE that is in a much better shape than it was in the mid-90's. By the time Hogan signed with WCW, people were already sick of him and wrestling was in it's coldest period ever, plus Hogan hadn't been a full time wrestler for two and a half years while Cena as I type this is STILL the top dog in WWE. Of course, after TNA put the world title on him, they would likely then book him in shitty angles that drew no money, but that's a different story.
Speaking of Hogan, there's another implausible scenario - Hulk Hogan returning and winning the WWE Title one last time.