I have loved watching Manning play but I think AD has to get the MVP. Peyton has had a great season but look at the QB stats for this season. He is comparable to Rodgers, Ryan, Brady, and maybe Brees. The only QB stat he leads in (AFAIK, could be wrong) is completion percentage.
It's a QB driven league we're in now. How many times have we heard that dominant RB's are a thing of the past, that it's a passing league and defenders are too big and the punishment too brutal? Yet Peterson has done it and done it on a team with no threat of passing whatsoever, and done so in dominant fashion. Over 2,000 yards, 6 yards per carry, almost 30 carries of 20+ yards. Ridiculous.
Next season Manning will have another ridiculous season and so will 5-6 other quarterbacks. A season like AD had is a rarity.