if you want top brakeing power, changing disks at same time is always good as the old disk will have its groves that the old pads fit into wher as the surface covered by the old pads will fit to the serface of the old disks, so when you put new pads on old disks they will not sit on the whole serface of the disk untill they have warn into the groves, which can really reduce the life of the pads, so new pads on new disks = perfect!