PowerFC you have to remove your management unit from the car, drop this in and dyno tune this. E-Manage is a piggyback which intercepts data going to and from management unit and make adjustments. Even this you have to dyno tune to get the car running properly.
stand alone ECM (PowerFC) is thousand times better than piggyback (E-Manage) when you want to milk every bit of power you got under the hood.
Now if do not have a turbo or supercharger added to your car, the gains with these units is negligible.
BTW Datalogit is a unit you plug to the PowerFC and to a computer, so it is easier to tune your PowerFC.