I'm a fan of Little Acorn but I don't expect him to be anything more than a mid to low top ten guy. His power alone will get him some good wins, but I don't see anything that'll elevate him to the top spot, regardless of whether a Klitcshko occupies that or not.
Mitchell is a exciting fighter, but he'll get exposed at the top level if he don't work on his defence. I think he's got a lot of good attributes going for him.