Lots of difference between the 2 cars mate it depends what you want!
Styling, engine, interior, there all different in one way or the other.
If you want a cheap RS to run about in and mod in due course then a Series 2 is for you, however if you want a timeless classic with heritage and class then its gotta be the Series 1, do be prepared to spend upwards of 3k for a decent one tho, as i'm finding out right now!
You'll probably pick up a cheap S2 that needs tidying and has probably been welded in the past for as little as 1500 quid tho mate, but remember - You get what you pay for!