It would be best to change out both. Which struts/shocks you use depends upon which springs you use. For mild lowering a set of E30 M3 springs will lower it about a half to 3/4 inch. H&R OE Sport springs lower the car one inch and H&R Sport Springs lower it 1.3 inches. Bilstein HD shocks can be used with stock, M3, or OE Sport springs. The Sport shocks (shorter) must be used with shocks that lower the car beyond one inch. If you use a different brand or wish to go lower than mentioned above you probably will require adjustible camber mounts, check with the vendor for proper match.