If you buy the correct SR (or CA) for your model of car...
CA18DET for 89-90
SR Redtop for 91-94
SR Blacktop for S14
or SR Blacktop for S15
it will be much easier to wire into the car. Nissan changed the wiring achitecture from model to model. A CA will easily drop into an 89-90 but an SR needs a little more wiring work done. Same for all the others as well. They all have different plugs on the harnesses and different colors for the wires as well. An S15 blacktop into a 89-90 should be a lot of fun to wire.
The retop also has the better flowing head than the blacktops. If you pplan to huck the factory turbo in the trash anyway... the red is the way to go. Also the S15 blacktop comes with that REALLY SHITTY 6-speed. MAJOR reliability problems if you get more than about 300hp out of the motor. It has the same size case as the 5 speed but with another gear stuffed in there. Thats why you see alot of the Drift S15's with S14 5-speeds with better internals in them