A key factor in the playoff semi at Hillsborough was the crowd. The Owls won't have that edge at Wembley and I think Barnsley are the better footballing side. I'll therefore go with the Tykes.
Whichever of Coventry or Luton win, and I'm not sure which one will, they'll be some major ground issues to be resolved before they enter the PL.
Agree that Stockport will sneak the League 2 playoff.