See my avatar,Jason all the way.
In reality Pennywise could probably manipulate Jason a lot like Freddy did,playing on his childhood fears. Despite being a huge Jason fan I can't let my fanboyism stand in the way. Take FVJ for example. Without human intervention Freddy would have dispatched jason pretty quickly in the boiler room dream sequence.
Pennywise could equally do the same.
But i'm still picking Jason.
btw,here's a link to a thread I did a while backPredator vs Jason