the SR20DET is starting to get up towards crazy cost wise and reliability wise when you are talking 350hp, where as the RB, 350hp is mainly a case of a slightly bigger turbo setup and good tune.
here in Australia a popular " mashup" is the RB25 DOHC head on the slightly earlier 3L RB30 block that came out in late '80s - early '90s Skyline and VL Holden Commodore, giving a fair bit more torque from bottom end with the performance of the twin cam head, not uncommon for 650-700hp+ from built and tuned motors.
Not too sure on the S15 but earlier models were able to swap hubs, calipers, rotors etc with skylines giving the benefit of 5 stud wheels, and cheaper cost bigger brake upgrades.