UGH. Dammit, I love both of these guys and don't want to see either lose in the first round. My head says Benoit but my heart says Ambrose.
Let's get the obvious out of the way: Benoit can out-wrestle Dean. Benoit can outpace Dean. Based on how his life ended, Benoit can probably out-crazy Dean too. Benoit has won more titles. Chris will probably win this (and maybe he should), so I'll focus on establishing why Dean could very well pull off the win.
When you look the advantages I gave Benoit over Ambrose, you know who also shares them? Triple H, a man who, on paper, is superior to Dean in every way. Yet Dean not only hung with Hunter, he basically pinned clean for the WWE title, thwarted only by a technicality. The same goes for Seth Rollins. The point is that Ambrose can beat world champions, and the times that he has, only technical BS got in the way of him actually being awarded the belt.
Ambrose's strongest attribute is the ability to absorb tremendous punishment. Benoit will go to town on Dean with various suplexes and chops, but Dean will just keep coming. Benoit tends to win matches via submission (Crossface/Sharpshooter) and Ambrose doesn't give up.
You can knock Dean for his inability to win the big one, but Benoit only won the big one once in Madison Square Garden. Other than his 4-5 month main event run, Benoit has been an upper mid-carder at most his entire career – the exact same spot as Ambrose. If anything, Dean has a FAR better shot of breaking into the main event and actually staying there than Benoit ever did. You MIGHT argue Dean is already a bigger star in terms of mainstream popularity and would go over on charisma alone.
Chris really isn't this insurmountable obstacle for Dean. Benoit took the fall against lesser guys more often than you'd think. I mean, the guy lost the US title in two straight falls to a rookie MVP of all people.
You know what, screw it. I'm voting Ambrose here. He won't win, but I've manage to convince myself that he could at least. I envision a completely spent Ambrose catching a lax Benoit, who thinks he's in total control, with a basic roll-up and completely surprising the technical master.