I think Arreola will stop him around the 6th or 7th round. But no matter who wins, he'll be in a position to challenge the Klitschkos or Haye in the near future.
I don't think Arreola has the stamina to win a 12 rd decision against Adamek. I think if Adamek is able to weather the first 4 or 5 rounds, he should win a decision, or possibly stop Cris late with accumulation. I'm kinda leaning towards Arreola stopping him around the 5th though.