Shields is going to take this fight. Jason Mayhem Miller doesn't have any skillset that will surprise Shields, and it will only be a matter of time before Shields gets this fight to the ground and submits Miller.
That being said, I do believe Shields is a Top Ten Welterweight, but in the lower half of the ranking. As Drogo says, Diaz would beat Shields, any of the American Kickboxing Academy clique would beat Shields [i.e. Kos, Swick, Fitch], Alves would overpower him, and Shields is no where near the competitive level of George St. Pierre.