s13 sr’s are 205 stock with a t25 turbo there was a s13 “black top” and a s13 “red top” called this by the color of there valve colors. no major changes just the blacktop came out of the years when the u.s s13’s went to the ka24de (dohc)
s14 sr’s are 220 stock with a t28 bb turbo theese also had variable valve timing
s15 motors are junk dont even worry about them. very expensive, very very weak trannys, harder to find parts for, and the wiring is a complete nightmare.
sr’s forged bottom end is good for roughly 400hp on stock internals
7lbs boost stock on all 3
kade’s are a great motor. torque monsters!!! imo more expensive to do it “right” tuned ecu, turbo set-up, either smic or fmic intercooler set up
so all in all the question has been argued millions of times. i have way way more info and i know tons about each motor. im in the middle of a s14 project. im putting a s14 sr in it.