OOOOOK, here goes:
In the S13 series, a Silvia is a coupe (S13 and PS13) with a notched back, it looks like this: http://english.auto.vl.ru/catalog/nissan/silvia/1992_1/21515/
In the same S13 shape is the RS13 and the RPS13 which are the lift backs like this: http://english.auto.vl.ru/catalog/nissan/180sx/1991_1/19671/
That's a 180sx, which NZ new was called a 200sx, despite being a 1.8 litre (CA18DET).
Pretty much _all_ 180 and 200sxs of that shape are turbo, I think I've only ever seen 1 non-turbo and that might have been a special order.
In the S14 shape I think they're exactly the same...at least no major differences, possibly trim spec or similar.
In the S15 shape they are the same car, they come with both badges I think.
For the years and more details:
http://www.nissansilvia.co.nz/tech/Silvia-180SX%20Table.htm