I've found the best way to figure out a question like that is imagine that you were writing an statless, UI less API for the application that was going to be called by various different clients, like a web service.
If the "manipulation" has to do with presentation in some UI of the objects represented by the models, then that sounds like controller. If its a "behavioral" feature of the objects themselves, or the type of manipulation that would be useful no matter what the presentation (for example a list of objects in some useful order based on the model properties) than I like to put that kind of thing in the model.
Hope that is somewhat helpful.