Del Rio will beat Edge... and it pains me to say that. While he should be a World Champion within the next year, he's going to get it too early come WM. He's not THAT entertaining to me or as great as some people want to believe. That being said, more recently (since he's won the Rumble), he's not anywhere near as bad or overrated as others want us to believe. He's overrated, but not that overrated. He's good in the ring, decent on the mic, plays his character to near perfection, and has a lot of charisma. The problem is he's now being pushed down our throats by WWE.
I don't believe anyone really buys Del Rio being worthy of main eventing WM (or being in one of the main events), only that they preference him over others and want him to be champion over a broken down in ring and character wise, Edge. That's fair but I still don't really buy him leading Smackdown as World Champion just yet. It's gonna happen though starting at WM, and I'm probably gonna get used to it, but for now, I just can't see it.
And as for people saying Miz will retain against Cena... that'd be the shocker of the year. The only way I could see that happening is if a Rock/Cena feud is in the works for the next few months and Rock costs Cena the match, but that's a long long LONG shot. On top of that unless WWE decides to let Edge/Del Rio main event (which won't happen), or Taker/Triple H main event (which I don't think should happen since a non-title match main eventing Mania should happen once in a blue-moon, not two years in a row), they're stuck with Cena/Miz. Because of that, Cena will have to be the face to send the fans home happy, and Del Rio will win the World Title to balance it all out earlier in the night.