I'm thinking Stiverne will pull off the upset. Neither man will have to go looking for one another so superior boxing skill won't come into deciding the winner. Arreola tends to be busier fighter and more aggressive he may leave himself open while coming in and get caught with a counter.
I realize Arreola isn't Floyd Mayweather, but he is a veteran fighter with something on the line in this fight. Hopefully he's savvy enough to realize that he can outland Stiverne and do plenty of damage without being blindly reckless. I'll go with the consensus of Arreola, KO6 or so.
Arreola will likely win by being more aggressive, as long as he can do it early. However, I can easily see Arreola not having taken Stiverne seriously, and so enter the ring not just out of shape, but ill-prepared. Stiverne, on the other hand, is probably taking this fight VERY seriously. I wouldn't be at all surprised if he KO's Arreola in the second half of the fight.