The S13 and RPS13 both have low curb weights, torqy and responsive motors, and pitiful wheel and tire packages. Combine this with most of the drift tuning styles and you end up with cars that like to swap ends more often than drift. I have a good bit of trouble pulling off smooth drifts in either. But, when you get a handle on them, they are quite rewarding.
i don't know the difference between the rps13 and the s13 but i have every silvia and i think they're all good. the s13 i use is set to Cudaman's settings. i've tried to change them but i think that they're the perfect settings. stick with them.
I found it to handle pretty nice, and able to sustain drifts quite easily, although I'm still kind of playing with it so don't expect the ultimate drift machine.
BTW, the RPS13 is the 180SX, the hatchback version of the 240SX. Actually in the States it's sold as the 240SX hatchback. Aside from the back-end, they're exactly the same as the coupe-style 240SX (Japan's 240SX). The Silvia (PS13) has the same back-end as the 240SX, but a unique front-end. Finally, the Sil-Eighty is the back-end of the 180SX with the front-end of a Silvia. Confused yet?