S14 has the T28 ball bearing turbo. You can boost alittle higher and you get more midrange torque. Of course the engine will have less miles as well.

I would say get the S13, because you can easily surpass the horsepower of an S14 motor with minor modifications and it will cost you less in the long run. Just make sure you get a good motor.

S13 red/black top will hold 14.7psi max on a T25
S14 black top will hold max 18psi (i think) on a T28

I think I've wrapped it up here.