I disagree. Campbell doesn't move well enough to play OLB in a 3-4, but he's also miscast having to hold the edge/rush the passer as a 3-4 DE as well. He'd be best suited as a LE, since he'd take on less blockers and be able to use his lateral quickness better.
I've watched some film on Miller, and while he's put up great numbers, teams consistently gain big yardage running right at him and he's consistently out of position. He's also not he biggest guy in the world and would have to add some size to play the 3-4 OLB in the NFL.
Bowers is a beast in the Julius Peppers mold, a guy you can build a defense around. He moves well enough that even if we didn't switch back to a 4-3, he could still play a rush backer position in the 3-4. Much better prospect than Miller IMO.