I was under the impression that the 180SX bodystyle was the lightest of the S13 chassis.
I've seen this quote on a few sites, dunno if it's true or not.
The second and third are the Silvia S13 and S14, which come in two different models: the turbocharged K's and the non-turbo Q's. Because of their high horse power and free-revving enginges they are excellent drifting cars. The third is the 180SX, related mechanically to the Silvia, the only difference is in the body style and the fact that is lighter and has a better front/rear balance ratio.