just wondering what the diffrences between them are or what years they were released?

S13 - 89-90(SOHC)91-94(DOHC) , 94's were only convertables and only automatic

S14 - 95-96 (Zenki)97-98(Kouki)

I am pretty sure I have this all straight I do not know much about the S15's as they are not available in the states, I am not positive on the years but they have the SR20DET with a few different Trim levels and stuff, I believe there is a Sequential transmission available too.

all i know for s15, 99-01, not sure on the end of it, but starts at 99.

The 180Sx S13 chassis went all the way into 1997.
The S13 Silvia ended with the beginning of the S14.
94's were only convertables and only automatic

I don't think that is correct. There are 94's that are not convertibles.

very true, there are 94's for convertibles and nonconvertibles.