S14 subframe is a direct fit and according to Epstein it helps to reduce toe in under compression which helps gain traction when exiting corners, but I'm betting that you need all the correct adjustable arms/ bearings or bushings to really benifit from that. I'd say stick the the S13 subframe, get all the neat arms and bushings, Z32 uprights and an S15 helical and rock out. Then if you come across an S14 or S15 subframe later on buy it and swap over everything since it all the arms are compatible with each chassis.

You dont really have to worry about upgrading the subframe unless your putting down some serious power and have traction problems in the first three gears on track.