S14>S13. Longer wheelbase = better stability. Yes it is also harder to make quick, tight transitions, but if that's all you want to do then get a Miata. The S14 also has a wider track, stiffer chassis, and hardly weighs more. S15>S14 for all the same reasons. If I could build any of them though, I'd stick with the S13 since I think all the other s-chassis are kinda ugly, especially the kouki s14 and s15 and you can swap all the parts to increase the track, seam weld to increase rigidity (but lose streetability), and make it 100x sessier looking IMO.