I'd go with the blacktop s13-sr , it has the revised 54C parts , and the high port head (for power building potential). The CA18DET , does look like it frees up more space inside the engine compartment and seems to be easier to work on but has less aftermarket support. Their are those that say it is more reliable then the SR , and less sensitive(more tolerant to abuse) then the SR's are (cannot verify that though). If the KA24DE had better pistons from the factory it would definetely be a excellent choice.