s15s are lighter, the jap spec is more powerful. But i see the prices for modifications are higher than skylines - in most cases anyway. R34 is rarer, more powerful, but heavier.
Both cars modded, look good and go good and both are around the same price i think. I wouldnt know what to go for if I was you. I suppose the jap spec s15 might have more parts as the 200sx was sold here...
Edit: didnt read the track work part - the silvia maybe better suited for the track too - depends on how you mod it. Im sure a lot of people here have skylines for the track. Im trying to make mine perfect for the track too - who knows how much that will set me back - but its fun anyway