The difference is going to be with how these allow weight transfer. 90/10 will allow more transfer to the rear and hold the nose higher for a longer period of time than the 70/30 will. If this is a dedicated street car, 90/10 are recommended. If you also drive on the street, the 70/30 will have better road manners. Having said that, we have customers running both combos on the street and are happy with them. It just depends on how much weight transfer you want at the track.