The S15 is a great car. If Skylines weren't so awesome the Silvia would be even MORE popular than it is.
A good upgrade from an R33, but less practical if you plan on having more than one passenger.
What everyone's said is true. It's good bang for buck and looks good. If you're not looking for huge power then the SR20 + 'box will be plenty of fun and reliable.
There isn't much difference stock for stock. They both have small factory turbos so make full boost very early between 2500-->3000RPM. BTW, the Silvia's T28 can be wound up to 18psi+, Skylines are limited to about 14psi due to ceramic exhaust turbine.
IMO the S15 is more responsive than the R33. It has 500cc's less and only 4cyl so it needs less boost to make power, plus it's lighter. Everyone's comments are correct, power delivery is different. The SR20 is punchy and hits hard in midrange. The RB25 is more torquey and gradual, like a surge of power.