Dont buy an S15 over an R33! Sure they are good cars and the SR20 is a good motor, but how could you ever choose a farty 4 cylinder over a smooth and awesome sounding straight 6?!?!
The inline six is the whole reason I love Skylines (along with BMW's). The R33 is my least favourite looking Skyline out of the R32/33/34, but I still do like the look of them, and they can look really tough if done right. The "fattness" of them can be an asset with some nice fat rims and lowered to the right level. Series 2/3 front ends look heaps better too so grab one of those if you can. Or spend a few more $$$ and get an R34!