i am one of the lucky people to have experienced owning both great cars, current ride is a s15, previous ride r33 GTST
in my view, the s15 beats the r33 mod for mod mainly due to the new technology being installed in the 2002 model i drive
it has r34 suspension aspects and the light weight makes it hammer through corners alot better
the power comes on alot faster and the car is stock, the r33 had power come on alot higher but kept going in the powerband, the s15 comes on fast and in higher revs loses out
i havent modded the s15 but feel it to be faster then my skyline, hell, my s15 can beat a 8psi s14 with a 3in full exhaust and i am still stock
it does come down to driver, the way the engine has been treated would also affect in some way i believe
i sold my skyline and the guy blew the engine up and also found a crack in the diff. lucky for me!!
so i choose s15, still love skylines however, even though there are like 500000000 of them out there
s15 looks alot better anyways, cant wait to start modifying it