I've thought about this more than someone with a life would (approximately ten minutes) and I've settled on this:
Offensively, Jonas Jerebko is better suited to be a Power Forward. As a Small Forward, he's not a good enough ball handler to be able to get to the rim. He has a fairly decent outside shot, but his release is very slow and I can't see him getting shots off against most SFs. His post game is not good enough to take advantage of his size. As a Power Forward, he's shown the ability to put the ball on the floor and get to the basket. His outside shot can stretch the defense and he has more time to get it off. Also, his quickness makes him a dangerous offensive rebounder.
Defensively, he's probably better suited to play the three. He can handle himself against some Power Forwards but he does need more strength. Against Small Forwards, he reminds me of Trevor Ariza defensively: long, athletic, and a phenomenal rebounder at that position.
I really like the idea of Austin Daye and Jerebko playing together with Daye being the "three" and Jerebko the "four." It's unconventional, but both of them could provide some real matchup problems.