go rb more hp more torque, can be modded to 500hp+ on stock internals with rb25+ but more money and no aluminum block like SR's same with ka they have iron blocks
if you dont like rb series engines i would go sr, more people have em and parts are more readily available than ca (but not ka)

IMHO

oh and the sr clips if u gonna buy one is expensive, u might as well go to xatracing.com and save up enough for them to build you a 450hp monster