I think this thread has generated some awesome points of view.
But it's labelled wrong- a fairer comparison is "S15 Vs R34" as Nissan produced them around the same time. The R33 should be compared to the S14. The 33 was made in '93, and you want to compare it to a '99 Silvia? Of course the S15 will beat it, it's newer and sleek.
From these labels i think Skylines have the edge.
At the end of the day, choosing a car should be an act of passion. You will be drawn to one or the other.
I don't know why, but my favourite Nissan is the R33 GTR, don't ask me to explain. I know the R34 GTR is better in every way but i'm drawn to the 33 more
They're in different classes also. You should compare classes (based on Price, target market, engine capacity/pistons, AWD/RWD/FWD) i.e. Skyline vs Supra, Integra vs Celica, Commodore vs Falcon.
Comparing Skyline vs Silvia is like comparing tits and ass.