The big difference is that piggybacks don't allow you to change anything but fuel, which is only one piece of the puzzle.

-Piggybacks for hack/budget setup
-ROM tune to do it right on a street or race car
-Standalone for crazy setups (bigger maps, meth/alc/water inj, onboard boost control). Also for fanboys with mild setups and deep pockets who think speed density is the shiz booyeee!