The spring rates are different, I believe. However, in terms of coilovers, you wouldn't notice any difference and it wouldn't effect handling. S14 coilovers springs, dampers and alignment parts will all fit S15s. In fact, in the case of the Apex stuff, there is a very good possibility that they were made for the S15 and have been applied to the S14. This is certainly the case with their FMIC. Bolts straight on an S15, but requires arsing about with the S14...