I had the A1 1.4 TFSI sport before the S1 so I've a very good idea
- S1 is obviously much, much quicker. The A1 was nippy but not exactly fast. Fine for motorway driving as long as you're willing to work the gears and get on the boost.
- Audi drive select is really nice to have on the S1, in Eco mode its quiet(er) and comfortable and in dynamic mode is a different animal completely, loud(er) and aggressive.
- The A1 was really good for economy, especially for a petrol that wasnt CoD. I used to get 40mpg around town and 50mpg+ on a motorway stint. To compare, the S1 gets 20-25mpg around town and 30-40mpg on the motorway. Not terrible but its only done 3000 miles so might 'bed in' and improve but I won't hold my breath.
- The S1
feels like a bigger car, probably because of the way it holds itself with the 4WD and the revised suspension. It feels more refined.
- The A1 is a lot cheaper. Obviously. But the S1 is worth it IMO, I don't care what the Golf GTI fanboys say.
- Having the Quattro is fantastic - the A1 handles well for a FWD car but this is next level and isn't phased by a bit of water on the road.
Anything else you want to know just ask.