ya i think the spec S one has the T25 turbo, and the spec R has the T28 turbo. i know that the s14 and s13 have the t25 and the s15 has the t28. i would guess that this is how it works for the spec S and R.
ya and some other things like
spec R comes with factory body kit
and it has a rear windshield viper plus there is a stock taco near the driver's window