see i asked this same question awhile back and got no strait answer, i thought the difference is in the compression, but its not.

s14= 8psi, 370's, 8.5to1
s15= 10.6psi, 480's, 8.8to1

might not be spot on on that, but i know its not 10to1. thats the n/a motor. the s15 motor is either 8.5 or 8.8, but not any higher. i know this cause i make 250hp at the wheels on my s13 sr on a s15 turbo on 10psi. the gtir t28 will get you right around the same power, but won't spool as fast.